written by: Emma Lloyd•edited by: Paul Arnold•updated: 6/11/2009
Antidepressants are medications used to treat mood disorders such as anxiety disorder, obsessive-compulsive disorder, and eating disorders, as well as depression. There are several different types, and each works slightly differently.
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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).
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Selective Serotonin Reuptake Inhibitors
Currently, the class of antidepressants called selective serotonin reuptake inhibitors (SSRIs) are the most prescribed. This class of antidepressants works on the basis that the depression is caused by serotonin deficiency, but does not target norepinephrine as other types do.
The SSRI family includes citalopram (Celexa), fluoexetine (Prozac), and paroxetine (Paxil). Some people on these drugs may experience side effects such as dry mouth, anxiety, insomnia, drowsiness, decreased appetite, and decreased sexual function. These symptoms are usually temporary. Some people experience a withdrawal syndrome when they cease taking an SSRI antidepressant, and typically all people taking an SSRI are advised to gradually reduce their dose.
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Serotonin-Norepinephrine Reuptake Inhibitors
Serotonin-norepinephrine reuptake inhibitors (SNRIs) are newer than both tricyclics and SSRIs, and work on both norepinephrine and serotonin. SNRIs include duloxetine (Cymbalta) and Venlafaxine (Effexor). This class of antidepressant has fairly similar side effects to the SSRIs, and may also require gradual dosage reduction to avoid withdrawal syndrome.
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Monoamine Oxidase Inhibitors
Monoamine oxidase inhibitors (MAOIs) may be prescribed for people who don’t experience good results with any other type of antibiotic. Taking MAOIs can be problematic, however, because certain food and drugs can cause potentially fatal drug interactions when taken together with the antidepressant.
MAOIs function by inhibiting the action of an enzyme called monoamine oxidase. The enzyme is responsible for a reaction which breaks down several neurotransmitters, including serotonin, norepinephrine, and dopamine. This class of antidepressant can be as effective as tricyclics, but due to their high risk of toxicity they are generally considered a last resort.
A new class of MAOIs, called reversible inhibitors of monoamine oxidase A (RIMAs), have a lower impact and are more selective in the way they work. RIMAs don’t require that patients monitor their diet in the same way, thus removing one of the restrictions that prevented MAOIs from being widely used.
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NaSSAs and NRIs
Two further types of antidepressants are noradrenergic and specific serotonergic antidepressants (NaSSAs), and norepinephrine reuptake inhibitors (NRIs).
NaSSAs are a newer class of antidepressants which increases neurotransmission of norepinephrine and serotonin by selectively blocking certain types of receptors. NRIs work by improving levels of noradrenaline available for synaptic use by blocking reuptake of this neurotransmitter. NRIs seem to be particularly effective at improving motivation and concentration.