SSRIs are the most common antidepressants: Prozac, Zoloft, Paxil, Lexapro, Wellbutrin, Buspar, Effexor. Though psychiatry argues that they're not addictive because you don't need increasing doses to get the desired effect, getting off of them can sure feel like withdrawal.
Here's what happens.
If you don't taper down off an SSRI slowly enough (and slowly enough means different things for different people), you may develop flu-like symptoms: headache, nausea, agitation, fatigue, concentration problems, vivid dreams, shock-like sensations, feeling like your skin is crawling or burning, vomiting, gastrointestinal problems, and even suicidal thoughts can occur.
Between 35% and 75% of people experience at least some of these symptoms if they suddenly stop or go down in dosage 10mg at a time. (The difference in percentages has to do with which study you're looking at.)
Reducing or stopping SSRIs with an extremely short half-life (the amount of time it takes for a medication to be half out of your body, usually used as a reference for how often a medication should be taken) too quickly can cause a kind of neurochemical rebound effect. The body stops adequately producing acetylcholine, a neurotransmitter important to neuron (brain cell) communication.
Interestingly, one of the two types of acetylcholine receptors is "nicotinic acetylcholine receptors"--they're stimulated by nicotine. When people try to quit smoking, they often have similar problems, because the acetylcholine is being affected in a similar way.
The effects can last from one week to several weeks, and in some people they're bad enough to necessitate taking time off from work. (Imagine explaining that one to your boss.)
Obviously in some cases the problem is that the person still needs the antidepressant. But sometimes the dosage just needs to be lowered more slowly--5mg at a time, or even 2.5 at a time.
The truly ironic thing is that SO many people just want to take a pill for their psychological problems. That's not good for two reasons.
Antidepressants are NOT ibuprofen, they're serious medications. I believe strongly in them, and in how much they can help people--when they're really needed. In those cases, dealing with the side effects--and the discontinuation problems when it's time to stop taking them--are worth it. For everyone else...not so much.