SSRIs vs SNRIs: how the two antidepressant classes compare

Antidepressants are medicines used mainly for depression but also for conditions including obsessive compulsive disorder, post-traumatic stress disorder, generalised anxiety disorder, panic disorder, social anxiety, bulimia, phobias, fibromyalgia or nerve pain, migraine and bedwetting in children, according to the NHS. The NHS says it is thought that they work by affecting chemicals in the brain called neurotransmitters, such as serotonin and noradrenaline, which are linked to mood and emotion.

SSRIs and SNRIs are two of the main types. SSRIs are the most widely used. SNRIs include the medicines duloxetine and venlafaxine. The NHS describes venlafaxine as a type of antidepressant called a selective noradrenaline reuptake inhibitor, thought to work by increasing the level of serotonin and noradrenaline.

In simple terms, SSRIs focus on serotonin, and SNRIs act on both serotonin and noradrenaline. That difference in action is the reason for most of the differences in how they are used.

The NHS states that SSRIs will usually be the first type prescribed because their side effects are generally easier to manage and they are less likely to cause serious problems if you take too much of the medicine. The choice of medicine is individual and is made with a prescriber.

The NHS also notes that some people find certain types work better for them, cause fewer side effects or cause side effects that are easier to deal with. If side effects are hard to manage or you do not feel the medicine is helping, it advises talking to your doctor, who can discuss options.

The NHS lists common antidepressant side effects including mood changes, feeling agitated or anxious, difficulty sleeping, weight gain, headaches, dry mouth, blurred vision, feeling or being sick, dizziness or drowsiness, loss of appetite, indigestion, bowel changes, sweating, heart rhythm problems and sexual problems. Many ease after a couple of weeks as the body gets used to the medicine, but some can continue.

For venlafaxine specifically, the NHS lists headaches, nausea and vomiting, dizziness or drowsiness, dry mouth, diarrhoea or constipation, sexual problems, insomnia and weight changes as common. Rare serious effects include allergic reaction, serotonin syndrome, very low mood with suicidal thoughts and unexplained muscle pain or weakness.

The NHS says suicidal thoughts or an urge to self-harm are not common but can occur when starting an antidepressant, and people aged 24 or under, or who have had these thoughts before, may be most at risk. Speak to a doctor straight away if this happens, and call 999 or go to A&E if you are in danger.

The NHS says most adults can take antidepressants, but they are usually not recommended for depression in people under 18, although sometimes they may be used alongside another treatment. Some can be taken in pregnancy or breastfeeding if the doctor judges that the benefits outweigh the risks.

For venlafaxine, the NHS says it may not be suitable if you have diabetes, epilepsy, heart disease, glaucoma, a bleeding disorder, or have ever had mania. It also warns that antidepressants can interact with NSAIDs, anticoagulants, other antidepressants and the herbal remedy St John's wort, and that alcohol is best avoided.

The NHS says people usually carry on with an antidepressant for at least 6 months after symptoms improve, and that stopping suddenly can cause withdrawal symptoms such as headaches, aching joints and muscles, nausea, sweating, dizziness, sleep problems, anxiety and "brain zaps". These usually begin within a few days and last a few weeks, but can be more severe or longer for some people. A doctor will usually agree a plan to reduce the dose gradually, over several weeks or months for venlafaxine. Do not switch or stop yourself.