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Selective Serotonin Reuptake Inhibitors (SSRI)

3. Prescribing points

SSRIs are generally taken once daily. Although depressive symptoms can improve significantly within two weeks it usually takes four to six weeks for maximal response to SSRIs.

An SSRI should generally be initiated at the lowest possible effective dose. Raising the dose may not always be beneficial and could increase adverse effects. If a higher dose is necessary, sufficient time (up to four weeks) should be allowed for an antidepressant response to develop before raising the dose. Dose increase may also be considered if symptoms recur in a patient established on an SSRI.

Careful supervision during early treatment can help to identify unusual changes in behaviour including worsening of symptoms, suicidal thoughts and behaviour, and aggression. Patients considered at risk of suicide or of causing harm should be referred to psychiatric services.

The patient should be closely questioned about the use of psychotropic drugs1 substances (including benzodiazepine2 use and alcohol intake) because they could affect the outcome of antidepressant treatment.

Some adverse effects of SSRIs commonly diminish with continuing use (for example, initial anxiety and nausea). Dose-related side effects of SSRIs may also improve by lowering the dose. If a particular SSRI is not well tolerated and adverse effects persist, it may be necessary to switch to another SSRI or to another class of antidepressant drugs. However, when switching, care is required to avoid excessive serotonin potentiation; the first SSRI should not be present in any clinically significant amount before the second SSRI is introduced.

SSRIs: comparison of licensed dose ranges and typical half lives

Note: full product information should be consulted for clinical decisions about use.







Licensed dose range

20–40 mg daily

10–20 mg daily

20–60 mg daily

50–300 mg daily

20–50 mg daily

50‑200 mg daily


36 hours

36 hours

Up to two weeks for active metabolite

15–20 hours

24 hours

26 hours

  1. A chemical substance which can alter brain function, resulting in changes in consciousness, cognition, perception, mood or behaviour.
  2. Class of medicines which have a sedating effect, most often used for anxiety and insomnia, but also used for muscle relaxation, managing convulsions, and as premedication for surgery and certain procedures. The class includes: chlordiazepoxide, diazepam, flurazepam, lorazepam, midazolam, nitrazepam, oxazepam, and tempazepam.