In the times of Chazal, people would grind up medicine as needed. Grinding is forbidden on Shabbos; therefore, Chazal enacted a gezeira that a person should not take medicine on Shabbos for a minor ailment. Even though it is not common nowadays for a consumer to crush his own medicine, the gezeira remains in full force. In general, it is forbidden for a person with a minor ailment to do anything on Shabbos which an observer would realize is being done for refuah. I asked Rav Heinemann, shlit”a, twenty questions regarding treating minor ailments on Shabbos. Below are the questions and his answers. Following that, I have added source material for those who wish to further understand these pesakim.
Q1: How bad does a headache or an allergy such as hay fever need to be in order to take medicine on Shabbos?
A person is generally not allowed to take medicine on […]