Darnitol is a fictional brand of headache medicine, as seen in the Energizer Bunny commercials.
In the commercial, a lady with a headache thinks about things that make loud noises, including a crying baby, a ringing telephone, and a barking dog, when an announcer says, "Tension headaches this bad need a pain reliever this good." He then tries to say, "New extra strength Darnitol, with easamigraine", The Energizer Bunny interrupts the commercial by appearing in the lady's thought balloon and knocking over the Darnitol, much to the lady's bewilderment.
Darnitol's name is a pun on the saying, "Darn it all!", and its ingredient, "Easamigraine", is a pun on "Ease a migraine".