Why is the letter Rx used on medical prescriptions? It came from the astrological sign Jupiter. In ancient times, Jupiter was thought to be most powerful of heavenly bodies in matter involving health and healing. Thus, it became customary to precede it with medical prescriptions with the sign of Jupiter as a mean of getting favorable results from the medicine. The X that you see is the elongated tail of the R crossed.
Another theory for Rx is it short for the word "recipere" or "recipe" which in Latin means to take