This group is one of three main types of drugs used to treat depression, and was the first to be introduced (in the 1950s). Tricyclic drugs work by blocking the re-uptake of the neurotransmitters serotonin and noradrenaline, thus increasing the amount of the neurotransmitters at the nerve cell’s receptors. In people with depression, fewer neurotransmitters than normal are released, resulting in a slowing of neural activities. The drugs have a sedative effect, which can be useful for depressives with sleep problems, and an antimuscarinic action which can cause dry mouth and constipation. Over dosage can produce COMA, fits and irregular heart rhythm (arrhythmia).