Antidepressant medications are used in the treatment of clinical depression, usually alongside other forms of therapy such as psychological or behavioral therapy. Antidepressants can be divided into four major classes which include:

• Tricyclic antidepressants
• Monoamine oxidase inhibitors (MAOIs)
• Serotonin reuptake inhibitors (SSRIs)
• Atypical antidepressants

Tricyclic antidepressants

Examples of drugs in this class include amitriptyline, clomipramine, desipramine, imipramine, nortriptyline and trimipramine. The tricyclic antidepressants are one of the oldest drug classes to be used in the treatment of depression.

The majority of these drugs act as serotonin-norepinephrine reuptake inhibitors by blocking the receptors for these neurotransmitters. This increases the concentration of serotonin and norepinephrine in the synaptic cleft and enhances neurotransmission.

The side effect profile of tricyclic antidepressants is less favourable compared to other newer agents which are therefore preferred over these older agents. Tricyclic antidepressants typically take around three to four weeks to show benefits. Side effects of the drugs include dry mouth, nausea, weight gain, constipation, difficulty in urination, loss of libido, cardiac side effects such as high blood pressure, abnormal heart rhythm and seizure.

Monoamine oxidase inhibitors (MAOIs)

Examples of drugs in this class include phenelzine, tranylcypromine and isocarboxazid. Monoamine oxidase is an enzyme that breaks down monoamine neurotransmitters. Drugs that inhibit the MAO enzyme therefore increase the availability of these neurotransmitters in the brain, enhancing neurotransmission and mood in people with depression. The MAOIs are also one of the oldest classes of antidepressants.

Serotonin reuptake inhibitors (SSRIs)

Examples of SSRIs include fluoxetine (Prozac), sertraline, fluvoxamine, paroxetine, citalopram and escitalopram. These drugs increase the synaptic concentration of the neurotransmitter serotonin by preventing its reuptake by the presynaptic neuron. SSRIs are currently the mainstay of treatment for depression and are preferred over the other classes of medications such as tricyclic antidepressants and MAOIs as they are thought to be more effective and cause fewer side effects.

Some of the side effects of SSRIs that may occur include increases in suicidal thoughts and behaviors, anxiety, restlessness, agitation, aggressiveness and hostility. Sudden cessation of SSRI treatment may also cause withdrawal syndrome. In addition, the treatment can cause minor gastrointestinal side effects such as nausea, vomiting, weight gain or loss, dry mouth, diarrhea, constipation, loss of sleep, tremors, headache, sweating, dizziness and loss of libido.

Atypical antidepressants

This is a group of relatively new and unique antidepressants that work in different ways to each other and do not fall into the typical categories of antidepressants. Examples in this class include bupropion, duloxetine, mirtazapine, venlafaxine, trazodone and nefazadone. However, like most antidepressants, these atypical versions work by affecting the balance of various neurotransmitters in the brain such as dopamine, serotonin and norepinephrine, in order to improve mood.

Like SSRIs, the atypical antidepressants are second-generation antidepressants and tend to cause less side effects than the first generation antidepressants. However, examples of some side effects that may be experienced with these drugs range from agitation and anxiety through to nausea, constipation and diarrhea to insomnia and sexual dysfunction, depending on the agent used.