Yes,Cyclobenzaprine have " anticholinergic" side effect, simply your body create nerve impulses which confused the smooth muscles present in the gastrointestinal tract, urinary tract, lungs, etc. So medication may cause stomach upset, heartburn, constipation,diarrhea, headache, dizziness, dry mouth in the first few days as your body adjusts to the medication. Also you may develop: Skin rash or itching, rapid heart rate, muscle stiffness, difficulty urinating,etc.. Medication is closely related to the tricyclic antidepressants, so lethargy is other side effect,