Prozac stays in the body a long time, whereas Paxil leaves the body very quickly. That means Prozac is much less likely to have withdrawal effects than Paxil, but then, so do most meds! If you're treating depression, they are theoretically interchangeable, but the reality is, everyone is different. Paxil seems to be stronger than other ssris, possibly because of its larger effect on choline receptors, which accounts for some its difficulties.