In ancient times Roman citizens worshiped Jupiter as a major deity. Thereafter during the middle ages physicians believed that planet Jupiter exercised great influence on the people’s health. So a tradition of writing ancient Roman sign of Jupiter started among the physicians. This classical tradition has since become widespread and it has become almost a rule to write ‘Rx’ to convey good wishes that, may Jupiter give speedy recovery. According to Roman legends Jupiter being the god of light, rain and lightening infused vitality in the universe. So it is but natural that Jupiter is considered as god who gives health and strength.
As shown in the picture the original sign of Jupiter is quite different and requires care to draw it accurately. Either the doctors found it difficult to draw or found it time consuming so it has been simplified over the period of time as ‘Rx’.
Medical prescription (Wikipedia)