The main difference between SSRIs and SNRIs is that SSRIs prevent the reuptake of serotonin and SNRIs prevent the reuptake of serotonin and norepinephrine. Serotonin and norepinephrine are substances that the brain uses to send messages from one nerve cell to another. They are also called neurotransmitters.

SSRI stands for selective serotonin reuptake inhibitor and SNRI stands for serotonin and norepinephrine reuptake inhibitor. SSRIs and SNRIs are two classes of medications that may be used for the treatment of depression, and sometimes for other conditions as well.

Common SSRIs include citalopram (Celexa), escitalopram (Lexapro), fluoxetine (Prozac, Sarafem), fluvoxamine (Luvox), paroxetine (Brisdelle, Paxil, Pexeva), sertraline (Zoloft), and there are others.

Common SNRIs include desvenlafaxine (Khedezla, Pristiq), duloxetine (Cymbalta, Irenka), venlafaxine (Effexor, Effexor XR), and there are others.

SSRIs and SNRIs work similarly, that is by preventing the reuptake of certain neurotransmitters in the brain (SSRIs: serotonin; SNRIs: serotonin and norepinephrine). Neurotransmitters are chemicals which nerve cells release to “talk” to other nerve cells. Neurotransmitters may also be called chemical messengers.

Reuptake is the process in which the neurotransmitter is reabsorbed back into the nerve cell once the stimulus has passed. Both SSRIs and SNRIs are a type of reuptake inhibitor, which means that they prevent the neurotransmitter from being reabsorbed back into the nerve cell that released it. This means that the neurotransmitter stays for a longer period in the gap between the two nerve cells (the gap is called the nerve synapse).

The benefits of antidepressants such as SSRIs and SNRIs are thought to be due to their effect on neurotransmitters, or the effect keeping them in the nerve synapse for longer has on the way the brain operates. The truth is, experts, don’t really know what causes depression, or even how antidepressants work. In the past, depression has been thought to be caused by a chemical imbalance in the brain or by a deficiency in serotonin. We know now it’s not that simple, but well-conducted trials have shown that antidepressants such as SSRIs and SNRIs are beneficial for depression which is why they are commonly prescribed.

What conditions do SSRIs and SNRIs treat?

SSRIs tend to be more commonly prescribed than SNRIs because they are effective at improving mood and tend to be less likely than some SNRIs to cause side effects.

Other conditions that SSRIs are approved to treat, in addition to depression, include:

Anxiety

Bulimia

Fibromyalgia

Hot flashes

Obsessive-compulsive disorder

Panic disorder

Post-Traumatic Stress Disorder

Premenstrual dysphoric disorder.

In addition to depression, some SNRIs are approved to treat:

Anxiety

Bipolar depression

Chronic muscle or joint pain

Diabetic neuropathy

Fibromyalgia

Low back pain

Osteoarthritis pain

Panic disorder

Social phobia.

Do SSRIs and SNRIs have different side effects?

SSRIs and SNRIs have similar side effects. Commonly reported side effects that may occur with either SSRIs or SNRIs include:

Blurred vision

Constipation

Dizziness

Drowsiness (fluoxetine is more likely to cause insomnia)

Dry mouth

Gastrointestinal upset (such as constipation, diarrhea, or nausea)

A Headache

Hot flushes

Insomnia

Nausea

Sexual dysfunction (such as reduced desire or erectile dysfunction).

SSRIs and SNRIs may also cause:

An increase in suicidal thoughts and behaviors, particularly in children and young adults under the age of 25 years. This is most likely to occur when starting therapy

Serotonin syndrome – this is caused by excessive levels of serotonin in the body and is more likely to occur with higher dosages of SSRIs or SNRIs, or when SSRIs or SNRIs are administered with other medications that also release serotonin (such as dextromethorphan, tramadol, and St. John's Wort). Symptoms include agitation, confusion, sweating, tremors, and a rapid heart rate

An increase in the risk of bleeding, especially if used with other medications that also increase bleeding risk.

Some SSRIs and most SNRIs have been associated with a discontinuation syndrome when they have been stopped suddenly. For this reason, it is best to withdraw all antidepressants slowly.

See SSRIs or SNRIs for a full list of side effects.