There are six main types of antidepressants, each working on different brain chemicals and carrying different side effect profiles. The most commonly prescribed today are SSRIs and SNRIs, but older classes like tricyclics and MAOIs are still used, and a newer class targeting glutamate has emerged for treatment-resistant depression. Understanding the differences can help you have a more informed conversation about which option fits your situation.
SSRIs: The Most Commonly Prescribed Type
Selective serotonin reuptake inhibitors are the first-line treatment for most people diagnosed with depression. They work by blocking the brain’s reabsorption of serotonin, a chemical messenger involved in mood regulation. This leaves more serotonin available in the gaps between nerve cells, which gradually improves mood signaling.
Six SSRIs are widely used: fluoxetine, fluvoxamine, paroxetine, sertraline, citalopram, and escitalopram. All are available as generics. They tend to cause fewer serious side effects than older antidepressants, which is a major reason they became the default starting point. That said, sexual side effects are notably common. In one large study, 73% of people taking SSRIs reported some form of sexual dysfunction, including reduced desire, difficulty with arousal, or trouble reaching orgasm. The rates varied by specific drug, ranging from about 58% with fluoxetine up to nearly 73% with citalopram.
SNRIs: Targeting Two Brain Chemicals
Serotonin and norepinephrine reuptake inhibitors block the reabsorption of both serotonin and norepinephrine, a second chemical messenger involved in alertness, energy, and pain signaling. This dual action makes SNRIs useful for more than depression alone.
Venlafaxine is approved for major depression, generalized anxiety, panic disorder, and social phobia. Duloxetine covers depression and anxiety but is also approved for diabetic nerve pain, fibromyalgia, musculoskeletal pain, and osteoarthritis. Desvenlafaxine is approved specifically for major depression, while milnacipran is approved only for fibromyalgia in the U.S. Sexual side effects with SNRIs are similar to SSRIs. Venlafaxine, for example, shows rates around 67%.
Atypical Antidepressants
Several antidepressants don’t fit neatly into the other categories. They’re grouped together as “atypical” because each works through a unique mechanism.
Bupropion blocks the reabsorption of dopamine and norepinephrine rather than serotonin. This distinction matters practically: bupropion has significantly lower rates of sexual side effects (roughly 10 to 25%) compared to SSRIs, and it’s actually associated with weight loss rather than weight gain. For people who find sexual dysfunction or weight gain intolerable on other antidepressants, bupropion is often a useful alternative. It can cause agitation, insomnia, and restlessness.
Mirtazapine works by increasing the release of norepinephrine and also boosts dopamine activity in the brain’s cortical regions. It tends to be sedating, which makes it helpful for people whose depression includes severe insomnia. Sexual side effects occur in about 24% of users, considerably lower than SSRIs. It can cause increased appetite and weight gain.
Trazodone acts on serotonin receptors and also blocks histamine receptors, which produces a strong sedative effect. It’s frequently prescribed at low doses as a sleep aid, even for people not taking it primarily for depression.
Tricyclic Antidepressants (TCAs)
Tricyclics were among the earliest antidepressants developed. They’re effective, but they’ve largely been replaced as a first choice because they cause more side effects than newer options. Common issues include drowsiness, blurred vision, constipation, dry mouth, difficulty urinating, and a drop in blood pressure when standing up (which can cause lightheadedness or fainting). These side effects stem from the drug’s broad activity across multiple receptor systems in the brain and body, not just the ones involved in mood.
TCAs are still used when someone hasn’t responded to SSRIs or SNRIs, or for specific conditions like certain types of chronic pain. They require more careful dosing because overdose risk is higher than with newer antidepressants.
MAOIs: Effective but Restrictive
Monoamine oxidase inhibitors work by blocking an enzyme that breaks down serotonin, norepinephrine, and dopamine. They’re effective antidepressants, but they come with a significant catch: that same enzyme also breaks down tyramine, a substance found naturally in many foods.
When the enzyme is blocked, tyramine can build up in your body and cause a dangerous spike in blood pressure that may require emergency treatment. This means you need to strictly avoid high-tyramine foods while taking an MAOI. The list includes aged cheeses (cheddar, Swiss, Parmesan, blue cheeses like Stilton and Gorgonzola, and brine-preserved cheeses like feta), fermented foods, overripe or spoiled foods, and even leftovers that have been stored too long. Caffeinated beverages may also need to be limited. Your prescriber will typically recommend eating only fresh foods.
Because of these dietary restrictions and drug interaction risks, MAOIs are generally reserved for people who haven’t responded to other antidepressant types. Interestingly, they have the lowest rates of sexual side effects of any antidepressant class, with one study finding a rate of just 3.9% for moclobemide, a reversible MAOI.
Rapid-Acting Antidepressants for Treatment Resistance
The newest class works on an entirely different brain system: glutamate, the brain’s primary excitatory chemical messenger. Esketamine, delivered as a nasal spray, is approved for adults with major depression who haven’t responded to at least two other antidepressants. Rather than gradually adjusting serotonin or norepinephrine over weeks, it blocks a type of glutamate receptor, which triggers a burst of new neural connections in brain regions that regulate mood and emotion.
Because of its powerful effects, esketamine must be administered in a certified healthcare setting with monitoring before and after each dose. It’s not a standalone treatment; it’s used alongside a conventional oral antidepressant.
How Long Antidepressants Take to Work
Most conventional antidepressants don’t provide relief immediately. Some effects can be detected within the first week, but meaningful improvement typically takes longer. Clinical guidelines recommend 4 to 8 weeks of treatment before deciding whether a particular medication is working.
Early improvement within the first two weeks is a good sign, but not seeing improvement early doesn’t mean the medication has failed. Research pooling data from multiple clinical trials found that 20 to 30% of people who showed no improvement at the two-week mark still responded well after 4 to 12 weeks. One study found 44% of people without early improvement still responded after 12 weeks. The overall probability of responding to any given antidepressant is around 50%, so finding the right fit sometimes takes patience or switching.
Side Effects Across Classes
Sexual dysfunction and weight changes are two of the most common reasons people want to switch antidepressants, so knowing which classes carry higher risk matters. SSRIs and venlafaxine (an SNRI) have the highest rates of sexual side effects, ranging from 58% to 73% depending on the specific drug. Bupropion (10 to 25%), mirtazapine (about 24%), and MAOIs (under 10%) carry substantially lower risk.
Weight gain is most associated with mirtazapine and some older tricyclics. Bupropion is the only antidepressant consistently linked to weight loss rather than gain. SSRIs and SNRIs fall somewhere in the middle, with effects varying by individual and by specific drug within the class.
The FDA’s Warning About Young Adults
All antidepressants carry an FDA black box warning, the agency’s most serious safety label, regarding suicidal thoughts in young people. The warning originated in 2004 after a meta-analysis of 372 clinical trials involving nearly 100,000 participants found higher rates of suicidal thinking among those assigned antidepressants compared to placebo. In an age-stratified analysis, the increased risk was statistically significant only in children and adolescents under 18. In 2006, the warning was extended to young adults up to age 25. The FDA also noted in 2007 that depression itself carries an increased risk of suicide, acknowledging the complexity of the issue. This warning doesn’t mean antidepressants are unsafe for young people. It means close monitoring is important during the early weeks of treatment, especially when starting or changing doses.