SSRIs, or selective serotonin re-uptake inhibitors like Zoloft, function by blocking the absorption of the neurotransmitter serotonin into the brain's cells, allowing it to flood the brain — and, the science goes, raise mood. But this serotonin flood looks like it also has an affect on appetite — though it's not the same for everybody, or indeed for most people who take it.
Wellbutrin, or bupropion, is an example of an atypical depressant, falling outside the normal lines of the industry. It's unique because it doesn't focus on serotonin levels. Instead, it stops the brain's re-uptake of norepinephrine and dopamine. And that combination seems to turn it in the opposite direction to SSRIs when it comes to appetite.
SNRIs like Cymbalta and Effexor try to have the best of both worlds: they disrupt the brain's re-uptake of both serotonin and norepineprhine. Interestingly, however, they often seem to fall into the same pattern as SSRIs, with up to 29.6 percent of patients experiencing weight gain while on an SNRI, according to the Journal of Pharmacology.
One of the possible explanations for this, and for all weight gain on anti-depressants, may be a side affect of a better mood: a 2006 study at the University of Bristol found that antidepressants actually improved the sense of taste in depressed people, who'd formerly been less sensitive to bitter, sweet and sour tastes. So it's theorized that antidepressant taste boosts, plus the artificial influence on cravings, may contribute to a greater appetite and more weight gain.
Tricyclics are one of the earlier discoveries in the antidepressant field, and are less prescribed than SSRIs or SNRIs, but they're still popular in their own way, and are often used for treating things other than depression, like ADHD. They're very similar to SNRIs, but they have a few differences: tricyclics have a different chemical structure, which means that they interact with the neurology of the brain differently — they effect more of the brain's neurotransmitters, and are associated with more nasty side affects.
These are the dinosaurs of the anti-depressant world, and they're exceptionally powerful drugs. They're the last resort, and are very rarely prescribed nowadays, because they do some pretty drastic things to the brain's neurotransmitters. It's pretty common lore in depression studies that, for all their blunt-fore action, they're actually less likely to cause weight gain than tricyclics or SNRIs — but we're not sure exactly why, or what it does to appetite.
One possible, and slightly upsetting, reason is that MAOI antidepressants are commonly associated with severe nausea — which obviously is a serious appetite suppressant. It likely also damages the brain's hormonal ability to listen to hormonal messages about appetite.
The Bottom Line
While a drug might sometimes cause a certain side-effect with appetite, there are often counter-cases where it does the precise opposite, because the balance of neurotransmitters in the brain is a highly individual thing. You never truly know how your body will react until you're taking a drug — which is why you shouldn't make a decision about which anti-depressant to take based on its potential influence on your appetite alone.