To name just a few:

Possible side effects of SSRIs may include, among others:

Nausea, vomiting or diarrhea.
Headache.
Drowsiness.
Dry mouth.
Insomnia.
Nervousness, agitation or restlessness.
Dizziness.
Sexual problems, such as reduced sexual desire, difficulty reaching orgasm or inability to maintain an erection (erectile dysfunction)

Obviously not all effect everyone the same but there are definitely potential side effects.