Can Uniphyllin Cause Muscle Stiffness?

Hi, I have been prescribed Uniphyllin 300mg for poorly-controlled asthma. I have been taking this drug twice daily (as directed) for about a week now and have been getting severe muscle stiffness which is keeping me awake at night. I understand that this is a side-effect of the medication, but is there anything I can do or take to lessen this stiffness and allow me to sleep?