If your Mounjaro pen failed or you can't tell whether the dose went in, don't start a second injection on your own: keep the pen and its carton, report it to Lilly online or at 1-800-545-5979 (9 a.m. to 5 p.m. ET, weekdays), and ask your prescriber or pharmacist how to handle this week's dose. If you already injected again and think you took too much, call your healthcare provider or Poison Help at 1-800-222-1222, or go to the nearest emergency room.