Tricyclics are the oldest class of antidepressant drugs, and while they are much less commonly prescribed with the advent of newer classes of drugs, they are still in use. This class of antidepressant works by blocking reuptake of serotonin and norepinephrine (noradrenaline). Serotonin is a neurotransmitter, a chemical which helps brain neurons transmit signals. Drugs which prevent reuptake of serotonin and other neurotransmitters work by increasing available levels of the chemicals in the synapses by preventing reuptake of the chemical by presynaptic neurons.
As the oldest class of antidepressants, tricyclic drugs have a somewhat broader range of side effects than most others. Possible side effects include increased heart rate, confusion, drowsiness, constipation, urine retention, blurred vision, and sexual dysfunction. Toxic doses are as low as ten times a normal dose, and can potentially cause fatal heart arrhythmia. Despite this, tricyclics remain useful because they are generally more effective than newer classes of antidepressants at treating people with major depression.
Types of tricyclic antidepressants include amitriptyline (Elavil, Endep) and Doxepin (Tofranil). Another class of tricyclics, called secondary amine tricyclic antidepressants, include nortriptyline (Aventyl, Pamelor) and protriptyline (Vivactil).