Based on the experiences shared, some common side effects of Fluoxetine (Prozac) include trouble falling asleep, headaches, and feeling groggy if a dose is missed. One user mentioned that when the medication wears off, it can result in a hard crash. As for how long it takes to start working, one user reported that it took between 10-14 days to feel the full effects of Prozac. However, everyone's experience with medication can be different.