What medication should I take to reduce headaches with the least number of side effects?

It depends to the. Cause of your headache. In general Tylenol (acetaminophen) would be a good choice. Tylenol (acetaminophen) rarely damages livers and kidneys, unless it is overused. I'd take 2 tylenol (acetaminophen) and call my doctor in the morning. Advil, aleve, naproxen can bother your stomach but are safe in moderate use.