 [ Symptoms ] what is black measles when i was young i had them now in my fifties i have a lot of health problems could it be because of them and what damage do they do to your body i know they have not been heard of in god know how long is there any way to know after all of these years after having them to get information on them [ 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