Amitriptyline belongs to a class of antidepressant drugs known as tricyclic antidepressants (TCAs). This class of drugs were one of the first used in the treatment of depression and work by raising levels of important mood controlling neurotransmitters such as serotonin and noradrenaline/norepinephrine in the brain. They do this by prolonging the time these neurotransmitters are present in the synapses (gaps) between nerve cells which means brain activity continues to be stimulated.

One of the drawbacks of TCAs is that they also block receptors for another neurotransmitters - acetylcholine. This leads to side-effects such as drowsiness and memory loss. This is one reason why TCAs are not used much today but have generally been replaced by newer drugs such as serotonin reuptake inhibitors (SSRIs) such as Prozac.

The 'side-effects' of amitriptyline however have been used in recent years for the treatment of conditions including sleep disorders, chronic fatigue syndrome (ME/CFS), and fibromyalgia. ME/CFS and fibromyalgia patients almost always have sleep disturbances and the anticholinergic effects of amitriptyline are used to help patients with this. The drug may also help with the pain of fibromyalgia.