 [ Symptoms ] does flexeril cause a fast heart rate [ Disease ] yes it can tachycardia rapid heart rate is listed as one of the more common side effects although i have to say that in my practice i have not seen this very often some people on flexeril will get a dry mouth tired irritable or nervous if your symptoms started at the same time that you started the flexeril there could definitely be a connection so please let your medical provider know so that an alternative muscle relaxant can be considered