SNRI (Serotonin Noradrenaline [Norepinephrine] Reuptake Inhibitor) antidepressants


There are a number of different SNRI antidepressants.

Brand name Chemical name
Efexor Venlafaxine
Serzone Nefazadone
Mirtazapin - closely related but not identical to SNRI

How SNRIs work

Serotonin and noradrenaline (also called norepinephrine) are neurotransmitters made by the cells in the brain. Neurotransmitters are chemicals used by brain cells to connect and communicate with each other. SNRI antidepressants slow the rate that the brain breaks down serotonin and noradrenaline. This leads to an increase in serotonin and noradrenaline activity in the brain. (Mirtazapine has a similar effect by a slightly different mechanism.) This has been shown to improve symptoms of depression and anxiety. It also reduces obsessions and compulsions in obsessive-compulsive disorders (OCD).

Side effects

Common side effects include:

  • Somnolence [Mirtazapine (Remeron/Avanza)]
  • Intensified dreaming
  • Mild nausea and headache which usually clear after a few days of use
  • Diarrhoea and abdominal cramping pain
  • Agitation and physical restlessness
  • Reduced sex drive and difficulty reaching a orgasm

The effect on weight is unclear.


  • Appetite reduction and weight loss most likely with Venlafaxine (Efexor) and Nefazadone (Serzone).
  • Weight gain most likely with Mirtazapine (Remeron/Avanza).

The effect on sleep is unpredictable.


  • Broken sleep is common with some of the SNRI medications (Venlafaxine (Efexor)).
  • Other SNRIs may cause sleepiness (Mirtazapine (Remeron/Avanza)).


Interactions with other drugs that affect serotonin can cause a serotonin syndrome. This syndrome involves fever, muscle shakes and seizures. It can be extremely serious and has caused deaths. Another serious problem that can occur is lowered sodium (salt) in the blood (Venlafaxine (Efexor)) SNRIs can also cause raised blood pressure. Lowered blood white cells can occur with Mirtazapine (Remeron/Avanza).


If changing between medications be careful that the overlap of medications does not cause too much serotonin activity. Fluoxetine leaves the body slowly and may exert its effect for more than three days.


SNRIs are usually safe. Overdose causes nausea and drowsiness. Deaths have been reported with very large overdoses.

Interactions with other drugs

Apart from the serotonin syndrome, some of the SNRI antidepressants slow the rate at which the body breaks down other medications. This may raise levels of medications such as:

  • Tricyclic antidepressants
  • Theophylline
  • Codeine
  • Beta-blockers (used for high blood pressure),
  • Alprazolam
  • Carbamazepine
  • Thioridazine
  • Terfenadine (Teldane)
  • Sumatriptan (migraine treatment)
  • Warfarin (anti-clotting medication)

It may be necessary to adjust the dose of other medications when a person is taking an SNRI.

When SSRIs must not be taken

SSRIs must not be taken at the same time as or soon after using Monoamine-oxidase inhibiting drugs (Parnate and Nardil).

Use in Pregnancy and Breast Feeding

There are no scientific studies of the effect of SNRIs on the human foetus. Animal studies show adverse effects at high doses. SNRI antidepressants should be used in pregnancy only if this is necessary for the health of the mother. SNRI antidepressants appear in breast milk. The manufacturers of Venlafaxine (Efexor) recommend that it Efexor be stopped if breast feeding is continuing. No information is available for Nefazadone (Serzone) or Mirtazapine (Remeron/Avanza).

Withdrawal effects

Antidepressants are not addictive. However, you may experience some withdrawal effects on stopping your antidepressant. Reactions vary between medications and between people. All SNRIs have been reported to produce some withdrawal effects - physical discomfort, restlessness and flu-like symptoms - if suddenly stopped. These symptoms are reduced by stopping the medication gradually (over 2 weeks from higher doses). Don't stop taking your medication without seeing a doctor first.

Dosage and time of dose

Often the starting dose does not need to be increased. However, sometimes higher doses are needed. Some people break down the medications faster than others and need higher doses.

Tablets are usually taken in the morning as many people find later doses interfere with sleep. However, this is very variable and some people find the medications make them sleepy and, so take them at night. The time of day does not change the effectiveness of the medication.

Food does not affect the absorption of Venlafaxine (Efexor) or Mirtazapine (Remeron/Avanza) so these medications can be taken before or after food. Nefazadone (Serzone) may be less absorbed so should be taken before food.

If a dose is missed, take it as soon as possible. If a night-time dose is missed do not take it in the morning but take only the ordinary night-time dose the next night.

Name Chemical name Common dose Most common time of dose
Efexor Venlafaxine 75 mgm - 300 mgm Morning
Nefazadone Serzone 200mgm - 600 mgm Morning

- closely related but not identical to SNRI

15 mgm - 45 mgm Evening

How long do you need to be on a medication?

You will need to take the antidepressant for at least 6 to 9 months after your symptoms disappear. Stopping your antidepressant too early increases your chances of suffering a recurrence of depression.

Are there problems with very long term use?

SSRIs have been in use for more than 7 years. Some people have taken them for that length of time. No extra problems have emerged beyond the common side effects mentioned above.

Last reviewed and updated: 8 February 2006