IMSAFE is the pilot's self-assessment checklist before every flight. The letters stand for six questions: Illness, Medication, Stress, Alcohol, Fatigue, and Eating/Emotion.
One honest "yes" is enough — you are sick, on medication, stressed, drunk or hungover, exhausted or unfed — to postpone the flight or hand it over to another pilot. IMSAFE checks the PILOT: it is a separate step alongside the weather check and the aircraft check.