Flexeril's main side effects are drowsiness, dry mouth and dizziness. Weight gain is included in the rare but perhaps possible side effects that a doctor should be aware of. These are generally side effects that showed up in low enough amounts during the clinical trials (less than 1%) that it wasn't clear if they were caused by the drug or if a small number of people in those trials just happened to gain weight during them. If you are experiencing sudden weight gain be sure to tell your physician about it. Good luck!