This depends mostly on what medication is being used, but it also varies a great deal from one person to another. The most common class of antidepressant medications used in the United States are the selective serotonin reuptake inhibitors (SSRIs), including fluoextine (Prozac), sertraline (Zoloft), paroxetine (Paxil), citalopram (Celexa) and escitalopram (Lexapro). Some of these, notably paroxetine, are well-known to cause weight gain. The others are more variable, and any physician who prescribes these medications, including myself, has seen people gain or lose weight on each of these drugs.
One commonly employed antidepressant medication, buproprion (Wellbutrin) tends to cause weight loss in users more often than weight gain. It is frequently used as an adjunct or “add-on” to the SSRI antidepressants because its properties are complementary in many respects. Buproprion itself is not approved by the FDA for weight loss, but there are ongoing trials of buproprion in combination with other medications that may lead to new options for weight management in the future. Buproprion is also the medication in Zyban, a prescription drug to assist with smoking cessation.
Although not strictly antidepressant medications, certain drugs known as “atypical antipsychotics” are often used as part of the medication regimen for those with severe depression. These include olanzepine (Zyprexa), risperidone (Risperdal), quetiapine (Seroquel) and aripiprazole (Abilify) among others. These medications are often very effective but tend to cause weight gain in many people.