Why do some medications cause headaches as a side effect?

In medicine, a side effect is an effect, whether therapeutic or adverse, that is secondary to the one intended; although the term is predominantly employed to describe adverse effects, it can also apply to beneficial, but unintended, consequences of the use of a drug.

This is a problem for people who have frequent, severe headaches. ... Medicines that can cause rebound headaches include: Nonprescription medicines, such as acetaminophen, aspirin, or nonsteroidal anti-inflammatory drugs (NSAIDs). Prescription pain medicines, such as codeine or hydrocodone. Hope this helps :) (Hope everything is okay too!!!)