Antidepressant drugs are a class of medications primarily used for treating various mood disorders, including major depression, anxiety disorders, and other related conditions. These medicines work by modulating the neurotransmitter balance within the brain, alleviating depressive symptoms. Antidepressants can be broadly categorized into several groups according to their mechanism of action and chemical structure: Selective Serotonin Reuptake Inhibitors (SSRIs), Serotonin-Norepinephrine Reuptake Inhibitors (SNRIs), Serotonin Receptor Antagonists, Tricyclic Antidepressants (TCAs), and Monoamine Oxidase Inhibitors (MAOIs).

1. Selective Serotonin Reuptake Inhibitors (SSRIs) are primarily prescribed for major depression and anxiety disorders. SSRIs block the reuptake of serotonin into presynaptic terminals. It is a neurotransmitter involved in mood regulation, leading to enhanced and prolonged serotonergic neurotransmission.
2. Serotonin-norepinephrine reuptake Inhibitors (SNRIs) act by inhibiting the reuptake of both serotonin and norepinephrine, enhancing the neurotransmission of these two vital mood-regulating chemicals. SNRIs were developed to improve the overall treatment response compared to SSRIs.
3. Serotonin Receptor Antagonists block the 5HT₂ family of receptors and serve as effective antidepressants.
4. Tricyclic Antidepressants (TCAs), despite their well-established efficacy, are not commonly used as first-line drugs due to their serious side effects. However, they continue to play a role in treating some insomnia and pain syndromes cases.
5. Monoamine Oxidase Inhibitors (MAOIs) have potency equivalent to TCAs but are infrequently used due to their toxicity and significant interactions with certain drugs and foods.

Antidepressant drugs are primarily used for treating major depression but also effectively treat various other mental health disorders. For instance, SSRIs and SNRIs are effective for generalized anxiety, panic, social anxiety, and obsessive-compulsive disorders. Despite their side effects, TCAs are still utilized for treating specific pain conditions and insomnia.

In conclusion, the landscape of antidepressant drugs is vast, with each class having its unique mechanism of action and therapeutic use. Understanding these various aspects is crucial for clinicians to make informed decisions when prescribing these agents. It's essential to consider the metabolism of these drugs, mediated mainly by hepatic CYPs, and potential drug interactions when choosing an appropriate antidepressant. As research progresses, further advancements in developing more effective and safer antidepressant agents can be expected.