SNRIs explained
What SNRIs are, how they work, and how they compare with SSRIs.
What SNRIs are
SNRIs, serotonin-norepinephrine reuptake inhibitors, are a class of antidepressant. The name describes what sets them apart. They act on two chemical messengers in the brain, serotonin and norepinephrine, rather than serotonin alone.
The main SNRIs are venlafaxine (Effexor), desvenlafaxine (Pristiq), duloxetine (Cymbalta), and levomilnacipran (Fetzima). Venlafaxine and duloxetine are the most widely used.
How they work
Serotonin and norepinephrine are chemical messengers that nerve cells use to pass signals to one another. After a cell releases one of these messengers, it reabsorbs much of it, a process called reuptake. SNRIs slow the reuptake of both, so more of each stays available between cells.
It is worth being honest about the limits of this explanation. The full picture of how SNRIs ease depression and anxiety is not known. What is clear is that the early change in signaling sets off slower adjustments in the brain over the following weeks, and those slower changes are thought to do the real work. That is why SNRIs take time to help rather than working the day you start them.
What SNRIs are used for
SNRIs are used for depression, generalized anxiety disorder, and other anxiety disorders.
Duloxetine in particular is also approved for several chronic pain conditions, including diabetic nerve pain, fibromyalgia, and chronic musculoskeletal pain. The norepinephrine action is part of why an SNRI can help with certain kinds of pain.
What they have in common
The medications in this class share a lot, and much of it is shared with SSRIs.
- A timeline of four to six weeks for the fuller effect.
- Early side effects that tend to arrive before the benefits.
- A shared set of common side effects: nausea, sweating, changes in sleep, and sexual side effects.
- Discontinuation symptoms if stopped abruptly, so they need a gradual taper planned with a prescriber.
- The antidepressant boxed warning about a possible increase in suicidal thoughts in people under 25, especially early in treatment.
- A risk of serotonin syndrome, a reaction caused by too much serotonin activity.
SNRIs are not addictive in the usual sense. They do not cause cravings or compulsive use. The body does adjust to them, which is why stopping should be gradual.
How they differ from SSRIs
The defining difference is the added action on norepinephrine. In practice this leads to two points worth knowing.
SNRIs can raise blood pressure. This effect is dose-related, and it is most associated with venlafaxine.
As a group, and venlafaxine especially, SNRIs are known for more noticeable discontinuation symptoms than many SSRIs. That makes a careful, gradual taper particularly important.
How they differ from each other
The SNRIs are not interchangeable.
Venlafaxine acts more like an SSRI at low doses, with its norepinephrine effect growing at higher doses.
Duloxetine carries the chronic pain approvals. It also comes with a caution about liver injury in people who drink heavily.
An SNRI is often chosen when an SSRI has not worked well enough, or when depression or anxiety comes alongside chronic pain.
The medications in this class
- Venlafaxine (Effexor). One of the most widely used SNRIs. It acts more like an SSRI at low doses, with its norepinephrine effect growing at higher doses, and it is known for more noticeable discontinuation symptoms.
- Duloxetine (Cymbalta). A widely used SNRI that is also approved for several chronic pain conditions, with a caution about liver injury in people who drink heavily.
- Desvenlafaxine (Pristiq). An SNRI closely related to venlafaxine, with simple once-daily dosing.
- Levomilnacipran (Fetzima). A newer SNRI used for depression.
PsychiatryRx has dedicated guides for venlafaxine and duloxetine, with more detail on uses, side effects, dosing, and what to expect.
Sources
This guide draws on current prescribing information and public health references. It is reviewed for clinical accuracy and updated as guidance changes.
- U.S. Food and Drug Administration. Prescribing information.
- MedlinePlus, U.S. National Library of Medicine.
- National Institute of Mental Health. Mental health medications.
This guide is for general education. It is not medical advice and is not a substitute for evaluation, diagnosis, or treatment by a qualified clinician. Never start, stop, or change a medication without talking to your prescriber. If you are in crisis or thinking about harming yourself, call or text 988 in the U.S. to reach the Suicide and Crisis Lifeline.