Selective serotonin re-uptake inhibitors or serotonin-specific reuptake inhibitors (SSRIs) are a class of drugs that are typically used as antidepressants in the treatment of major depressive disorder and anxiety disorders.
The exact mechanism of SSRIs is unknown. SSRIs are believed to increase the extracellular level of the neurotransmitter serotonin by limiting its reabsorption into the presynaptic cell, increasing the level of serotonin in the synaptic cleft available to bind to the postsynaptic receptor. They have varying degrees of selectivity for the other monoamine transporters, with pure SSRIs having only weak affinity for the norepinephrine and dopamine transporters.
SSRIs are the most widely prescribed antidepressants in many countries. The efficacy of SSRIs in mild or moderate cases of depression has been disputed.
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Medical uses
The main indication for SSRIs is major depressive disorder (also called "major depression", "clinical depression" and often simply "depression"). SSRIs are frequently prescribed for anxiety disorders, such as social anxiety disorder, panic disorders, obsessive-compulsive disorder (OCD), eating disorders, chronic pain and occasionally, for posttraumatic stress disorder (PTSD). They are also frequently used to treat depersonalization disorder, although generally with poor results.
Depression
Antidepressants are recommended by the National Institute for Health and Care Excellence (NICE) as a first-line treatment of severe depression and for the treatment of mild-to-moderate depression that persists after conservative measures such as cognitive therapy. They recommend against their routine use in those who have chronic health problems and mild depression.
There has been controversy regarding the efficacy of antidepressants in treating depression depending on its severity and duration.
- Two meta-analyses published in 2008 (Kirsch) and 2010 (Fournier) found that in mild and moderate depression, the effect of SSRIs is small or none compared to placebo, while in very severe depression the effect of SSRIs is between "relatively small" and "substantial". The 2008 meta-analysis combined 35 clinical trials submitted to the Food and Drug Administration (FDA) before licensing of four newer antidepressants (including the SSRIs paroxetine and fluoxetine, the non-SSRI antidepressant nefazodone, and the serotonin and norepinephrine reuptake inhibitor (SNRI) venlafaxine). The authors attributed the relationship between severity and efficacy to a reduction of the placebo effect in severely depressed patients, rather than an increase in the effect of the medication. Some researchers have questioned the statistical basis of this study suggesting that it underestimates the effect size of antidepressants.
- A 2010 comprehensive review conducted by NICE concluded that antidepressants have no advantage over placebo in the treatment of short-term mild depression, but that the available evidence supported the use of antidepressants in the treatment of dysthymia and other forms of chronic mild depression.
- A 2012 meta-analysis of fluoxetine and venlafaxine concluded that statistically and clinically significant treatment effects were observed for each drug relative to placebo irrespective of baseline depression severity.
- In 2014 the U.S. FDA published a systematic review of all antidepressant maintenance trials submitted to the agency between 1985 and 2012. The authors concluded that maintenance treatment reduced the risk of relapse by 52% compared to placebo, and that this effect was primarily due to recurrent depression in the placebo group rather than a drug withdrawal effect.
There does not appear to be a big difference in the effectiveness between medications in the second generation antidepressants (SSRIs and SNRIs).
In children there are concerns around the quality of the evidence on the meaningfulness of benefits seen. If a medication is used, fluoxetine appears to be first line.
Generalized anxiety disorder
SSRIs are recommended by the National Institute for Health and Care Excellence (NICE) for the treatment of generalized anxiety disorder (GAD) that has failed to respond to conservative measures such as education and self-help activities. GAD is a common disorder of which the central feature is excessive worry about a number of different events. Key symptoms include excessive anxiety about multiple events and issues, and difficulty controlling worrisome thoughts that persists for at least 6 months.
Antidepressants provide a modest-to-moderate reduction in anxiety in GAD, and are superior to placebo in treating GAD. The efficacy of different antidepressants is similar.
Obsessive compulsive disorder
SSRIs are a second line treatment of adult obsessive compulsive disorder (OCD) with mild functional impairment and as first line treatment for those with moderate or severe impairment. In children, SSRIs can be considered a second line therapy in those with moderate-to-severe impairment, with close monitoring for psychiatric adverse effects. SSRIs are efficacious in the treatment of OCD; patients treated with SSRIs are about twice as likely to respond to treatment as those treated with placebo. Efficacy has been demonstrated both in short-term treatment trials of 6 to 24 weeks and in discontinuation trials of 28 to 52 weeks duration.
Eating disorders
Anti-depressants are recommended as an alternative or additional first step to self-help programs in the treatment of bulimia nervosa. SSRIs (fluoxetine in particular) are preferred over other anti-depressants due to their acceptability, tolerability, and superior reduction of symptoms in short-term trials. Long-term efficacy remains poorly characterized.
Similar recommendations apply to binge eating disorder. SSRIs provide short-term reductions in binge eating behavior, but have not been associated with significant weight loss.
Clinical trials have generated mostly negative results for the use of SSRIs in the treatment of anorexia nervosa. Treatment guidelines from the National Institute of Health and Clinical Excellence recommend against the use of SSRIs in this disorder. Those from the American Psychiatric Association note that SSRIs confer no advantage regarding weight gain, but that they may be used for the treatment of co-existing depressive, anxiety, or obsessive-compulsive disorders.
Stroke recovery
SSRIs have been used in the treatment of stroke patients, including those with and without symptoms of depression. A recent meta-analysis of randomized, controlled clinical trials found a statistically significant effect of SSRIs on dependence, neurological deficit, depression, and anxiety. There was no statistically significant effect on death, motor deficits, or cognition.
Premature ejaculation
SSRIs are effective for the treatment of premature ejaculation. Chronic administration is more efficacious than on demand use.
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Adverse effects
Side effects vary among the individual drugs of this class. However, certain types of adverse effects are found broadly among most if not all members of this class:
- increased risk of bone fractures by 1.7 fold
- akathisia
- suicidal ideation (thoughts of suicide) (see below)
- photosensitivity
Sexual dysfunction
SSRIs can cause various types of sexual dysfunction such as anorgasmia, erectile dysfunction, diminished libido, genital numbness, and sexual anhedonia (pleasureless orgasm). Sexual problems are common with SSRIs. Poor sexual function is also one of the most common reasons people stop the medication.
Occasionally symptoms of sexual dysfunction may persist after discontinuation of SSRIs.
The mechanism by which SSRIs cause sexual side effects is not well understood as of 2015. The range of possible mechanisms includes (1) nonspecific neurological effects (e.g., sedation) that globally impair behavior including sexual function; (2) specific effects on brain systems mediating sexual function; (3) specific effects on peripheral tissues and organs, such as the penis, that mediate sexual function; and (4) direct or indirect effects on hormones mediating sexual function. It is probable that antidepressants impact several of these physiologic substrates of sexual function. Animal research and data from studies in human subjects suggest that sexual behavior and function are enhanced by increases in brain dopaminergic function and inhibited by increases in brain serotonergic function. The latter observation is consistent with the association of serotonergic antidepressants (which increase serotonergic transmission) with sexual dysfunction.
A number of (non-SSRI) drugs are not associated with sexual side effects (such as bupropion, mirtazapine, tianeptine, agomelatine and moclobemide.)
There is no FDA-approved treatment for SSRI-induced sexual dysfunction and there has been a lack of randomized, placebo-controlled, double-blind studies of potential treatments. There is evidence for the following management strategies: for erectile dysfunction, the addition of a PDE5 inhibitor such as sildenafil; for decreased libido, possibly adding or switching to bupropion; and for overall sexual dysfunction, switching to nefazodone.
Several small studies have suggested that SSRIs may adversely affect semen quality.
Cardiac
SSRIs do not appear to affect the risk of coronary heart disease (CHD) in those without a previous diagnosis of CHD. A large cohort study suggested no substantial increase in the risk of cardiac malformations attributable to SSRI usage during the first trimester of pregnancy. A number of large studies of people without known pre-existing heart disease have reported no EKG changes related to SSRI use. The recommended maximum daily dose of citalopram and escitalopram was reduced due to concerns with QT interval prolongation. In overdose, fluoxetine has been reported to cause sinus tachycardia, myocardial infarction, junctional rhythms and trigeminy. Some authors have suggested electrocardiographic monitoring in patients with severe pre-existing cardiovascular disease who are taking SSRIs.
Bleeding
SSRIs interact with anticoagulants, like warfarin and aspirin. This includes an increased risk of GI bleeding, and post operative bleeding. The relative risk of intracranial bleeding is increased, but the absolute risk is very low. SSRIs are known to cause platelet dysfunction. This risk is greater in those who are also on anticoagulants, antiplatelet agents and NSAIDs (nonsteroidal anti-inflammatory drugs), as well as with the co-existence of underlying diseases such as cirrhosis of the liver or liver failure.
Discontinuation syndrome
Serotonin reuptake inhibitors should not be abruptly discontinued after extended therapy, and whenever possible, should be tapered over several weeks to minimize discontinuation-related symptoms which may include nausea, headache, dizziness, chills, body aches, paresthesias, insomnia, and electric shock-like sensations. Paroxetine may produce discontinuation-related symptoms at a greater rate than other SSRIs, though qualitatively similar effects have been reported for all SSRIs. Discontinuation effects appear to be less for fluoxetine, perhaps owing to its long half-life and the natural tapering effect associated with its slow clearance from the body. One strategy for minimizing SSRI discontinuation symptoms is to switch the patient to fluoxetine and then taper and discontinue the fluoxetine.
Suicide risk
Children and adolescents
Meta analyses of short duration randomized clinical trials have found that SSRI use is related to a higher risk of suicidal behavior in children and adolescents. For instance, a 2004 U.S. Food and Drug Administration (FDA) analysis of clinical trials on children with major depressive disorder found statistically significant increases of the risks of "possible suicidal ideation and suicidal behavior" by about 80%, and of agitation and hostility by about 130%. According to the FDA, the heightened risk of suicidality is within the first one to two months of treatment. The National Institute for Health and Care Excellence (NICE) places the excess risk in the "early stages of treatment". The European Psychiatric Association places the excess risk in the first two weeks of treatment and, based on a combination of epidemiological, prospective cohort, medical claims, and randomized clinical trial data, concludes that a protective effect dominates after this early period. A 2012 Cochrane review found that at six to nine months, suicidal ideation remained higher in children treated with antidepressants compared to those treated with psychological therapy.
A recent comparison of aggression and hostility occurring during treatment with fluoxetine to placebo in children and adolescents found that no significant difference between the fluoxetine group and a placebo group. There is also evidence that higher rates of SSRI prescriptions are associated with lower rates of suicide in children, though since the evidence is correlational, the true nature of the relationship is unclear.
In 2004, the Medicines and Healthcare products Regulatory Agency (MHRA) in the United Kingdom judged fluoxetine (Prozac) to be the only antidepressant that offered a favorable risk-benefit ratio in children with depression, though it was also associated with a slight increase in the risk of self-harm and suicidal ideation. Only two SSRIs are licensed for use with children in the UK, sertraline (Zoloft) and fluvoxamine (Luvox), and only for the treatment of obsessive-compulsive disorder. Fluoxetine is not licensed for this use.
Adults
It is unclear whether or not SSRIs affect the risk of suicidal behavior for adults.
- A 2005 meta-analysis of drug company data found no evidence that SSRIs increased the risk of suicide; however, important protective or hazardous effects could not be excluded.
- A 2005 review observed that suicide attempts are increased in those who use SSRIs as compared to placebo and compared to therapeutic interventions other than tricyclic antidepressants. No difference risk of suicide attempts was detected between SSRIs versus tricyclic antidepressants.
- On the other hand, a 2006 review suggests that the widespread use of antidepressants in the new "SSRI-era" appears to have led to a highly significant decline in suicide rates in most countries with traditionally high baseline suicide rates. The decline is particularly striking for women who, compared with men, seek more help for depression. Recent clinical data on large samples in the US too have revealed a protective effect of antidepressant against suicide.
- A 2006 meta-analysis of random controlled trials suggests that SSRIs increase suicide ideation compared with placebo. However, the observational studies suggest that SSRIs did not increase suicide risk more than older antidepressants. The researchers stated that if SSRIs increase suicide risk in some patients, the number of additional deaths is very small because ecological studies have generally found that suicide mortality has declined (or at least not increased) as SSRI use has increased.
- An additional meta-analysis by the FDA in 2006 found an age-related effect of SSRI's. Among adults younger than 25 years, results indicated that there was a higher risk for suicidal behavior. For adults between 25 and 64, the effect appears neutral on suicidal behavior but possibly protective for suicidal behavior for adults between the ages of 25 and 64. For adults older than 64, SSRI's seem to reduce the risk of both suicidal behavior.
Pregnancy and breastfeeding
SSRI use in pregnancy has been associated with a variety of risks with varying degrees of proof of causation. As depression is independently associated with negative pregnancy outcomes, determining the extent to which observed associations between antidepressant use and specific adverse outcomes reflects a causative relationship has been difficult in some cases. In other cases, the attribution of adverse outcomes to antidepressant exposure seems fairly clear.
SSRI use in pregnancy is associated with an increased risk of spontaneous abortion of about 1.7-fold. Use is also associated preterm birth.
A systematic review of the risk of major birth defects in antidepressant-exposed pregnancies found a small increase (3% to 24%) in the risk of major malformations and a risk of cardiovascular birth defects that did not differ from non-exposed pregnancies. A study of fluoxetine-exposed pregnancies found a 12% increase in the risk of major malformations that just missed statistical significance. Other studies have found an increased risk of cardiovascular birth defects among depressed mothers not undergoing SSRI treatment, suggesting the possibility of ascertainment bias, e.g. that worried mothers may pursue more aggressive testing of their infants. Another study found no increase in cardiovascular birth defects and a 27% increased risk of major malformations in SSRI exposed pregnancies.
The FDA issued a statement on July 19, 2006 stating nursing mothers on SSRIs must discuss treatment with their physicians. However, the medical literature on the safety of SSRIs has determined that some SSRIs like Sertraline and Paroxetine are considered safe for breastfeeding.
Neonatal abstinence syndrome
Several studies have documented neonatal abstinence syndrome, a syndrome of neurological, gastrointestinal, autonomic, endocrine and/or respiratory symptoms among a large minority of infants with intrauterine exposure. These syndromes are short-lived, but insufficient long-term data is available to determine whether there are long-term effects.
Persistent pulmonary hypertension
Persistent pulmonary hypertension (PPHN) is a serious and life-threatening, but very rare, lung condition that occurs soon after birth of the newborn. Newborn babies with PPHN have high pressure in their lung blood vessels and are not able to get enough oxygen into their bloodstream. About 1 to 2 babies per 1000 babies born in the U.S. develop PPHN shortly after birth, and often they need intensive medical care. It is associated with about a 25% risk of significant long-term neurological deficits. A 2014 meta analysis found no increased risk of persistent pulmonary hypertension associated with exposure to SSRI's in early pregnancy and a slight increase in risk associates with exposure late in pregnancy; "an estimated 286 to 351 women would need to be treated with an SSRI in late pregnancy to result in an average of one additional case of persistent pulmonary hypertension of the newborn.". A review published in 2012 reached conclusions very similar to those of the 2014 study.
Neuropsychiatric effects in offspring
According to a 2015 review available data found that "some signal exists suggesting that antenatal exposure to SSRIs may increase the risk of ASDs (autism spectrum disorders)" even though a large cohort study published in 2013 and a cohort study using data from Finland's national register between the years 1996 and 2010 and published in 2016 found no significant association between SSRI use and autism in offspring. The 2016 Finland study also found no association with ADHD, but did find an association with increased rates of depression diagnoses in early adolescence.
Overdose
SSRIs appear safer in overdose when compared with traditional antidepressants, such as the tricyclic antidepressants. This relative safety is supported both by case series and studies of deaths per numbers of prescriptions. However, case reports of SSRI poisoning have indicated that severe toxicity can occur and deaths have been reported following massive single ingestions, although this is exceedingly uncommon when compared to the tricyclic antidepressants.
Because of the wide therapeutic index of the SSRIs, most patients will have mild or no symptoms following moderate overdoses. The most commonly reported severe effect following SSRI overdose is serotonin syndrome; serotonin toxicity is usually associated with very high overdoses or multiple drug ingestion. Other reported significant effects include coma, seizures, and cardiac toxicity.
The SSRIs, in decreasing toxicity in overdose, can be listed as follows:
- Citalopram (due to the potential for QT interval prolongation)
- Fluvoxamine
- Escitalopram
- Paroxetine
- Sertraline
- Fluoxetine
Contraindications and drug interactions
The following drugs may precipitate serotonin syndrome in people on SSRIs:
- Linezolid
- Monoamine oxidase inhibitors (MAOIs) including moclobemide, phenelzine, tranylcypromine, selegiline and methylene blue
- Lithium
- Sibutramine
- MDMA (ecstasy)
- Dextromethorphan
- Tramadol
- Pethidine/meperidine
- St. John's wort
- Yohimbe
- Tricyclic antidepressants (TCAs)
- Serotonin-norepinephrine reuptake inhibitors (SNRIs)
- Buspirone
- Triptan
- Mirtazapine
Painkillers of the NSAIDs drug family may interfere and reduce efficiency of SSRIs and may compound the increased risk of gastrointestinal bleeds caused by SSRI use. NSAIDs include:
- Aspirin
- Ibuprofen (Advil, Nurofen)
- Naproxen (Aleve)
There are a number of potential pharmacokinetic interactions between the various individual SSRIs and other medications. Most of these arise from the fact that every SSRI has the ability to inhibit certain P450 cytochromes.
Legend:
0 -- no inhibition.
+ -- mild inhibition.
++ -- moderate inhibition.
+++ -- strong inhibition.
List of agents
Drugs in this class include:
Most common:
- citalopram
- escitalopram
- fluoxetine
- fluvoxamine
- paroxetine
- sertraline
Other:
- dapoxetine
- indalpine (discontinued)
- zimelidine (discontinued)
- cericlamine (reached phase III; discontinued in 1999)
- panuramine
Related agents
SSRIs form a subclass of serotonin uptake inhibitors, which includes other non-selective inhibitors as well. The serotonergic serotonin-norepinephrine reuptake inhibitors and serotonin-norepinephrine-dopamine reuptake inhibitors are also commonly used as antidepressants.
Mechanism of action
In the brain, messages are passed from a nerve cell to another via a chemical synapse, a small gap between the cells. The presynaptic cell that sends the information releases neurotransmitters including serotonin into that gap. The neurotransmitters are then recognized by receptors on the surface of the recipient postsynaptic cell, which upon this stimulation, in turn, relays the signal. About 10% of the neurotransmitters are lost in this process; the other 90% are released from the receptors and taken up again by monoamine transporters into the sending presynaptic cell, a process called reuptake.
SSRIs inhibit the reuptake of serotonin. As a result, the serotonin stays in the synaptic gap longer than it normally would, and may repeatedly stimulate the receptors of the recipient cell. In the short run, this leads to an increase in signaling across synapses in which serotonin serves as the primary neurotransmitter. On chronic dosing, the increased occupancy of pre-synaptic serotonin receptors signals the pre-synaptic neuron to synthesize and release less serotonin. Serotonin levels within the synapse drop, then rise again, ultimately leading to downregulation of post-synaptic serotonin receptors. Other, indirect effects may include increased norepinephrine output, increased neuronal cyclic AMP levels, and increased levels of regulatory factors such as BDNF and CREB. Owing to the lack of a widely accepted comprehensive theory of the biology of mood disorders, there is no widely accepted theory of how these changes lead to the mood-elevating and anti-anxiety effects of SSRIs.
Pharmacogenetics
Large bodies of research are devoted to using genetic markers to predict whether patients will respond to SSRIs or have side effects that will cause their discontinuation, although these tests are not yet ready for widespread clinical use.
SSRIs versus TCAs
SSRIs are described as 'selective' because they affect only the reuptake pumps responsible for serotonin, as opposed to earlier antidepressants, which affect other monoamine neurotransmitters as well, and as a result, SSRIs have fewer side effects.
There appears to be no significant difference in effectiveness between SSRIs and tricyclic antidepressants, which were the most commonly used class of antidepressants before the development of SSRIs. However, SSRIs have the important advantage that their toxic dose is high, and, therefore, they are much more difficult to use as a means to commit suicide. Further, they have fewer and milder side effects. Tricyclic antidepressants also have a higher risk of serious cardiovascular side effects, which SSRIs lack.
SSRIs act on signal pathways such as cAMP (Cyclic AMP) on the postsynaptic neuronal cell, which leads to the release of Brain-Derived Neurotrophic Factor (BDNF). BDNF enhances the growth and survival of cortical neurons and synapses.
Society and culture
Controversy
A study examining publication of results from FDA-evaluated antidepressants concluded that those with favorable results were much more likely to be published than those with negative results.
David Healy has argued that warning signs were available for many years prior to regulatory authorities moving to put warnings on antidepressant labels that they might cause suicidal thoughts. At the time these warnings were added, others argued that the evidence for harm remained unpersuasive and others continued to do so after the warnings were added.
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