Researchers at the University of Missouri are warning people not to take over-the-counter medications such as acetaminophen (Tylenol) or aspirin soon after getting the H1N1 flu shot. They say it could limit the effectiveness of the vaccine by interfering with the body’s production of antibodies, which are in required to fight the virus. So grin and bear the soreness you may feel around the injection site—it may save you from the flu later.