Caffeine: A stimulant compound found naturally in coffee, tea, cocoa (chocolate), and kola nuts (cola) and added to soft drinks, foods, and medicines. Caffeine can cause anxiety, insomnia, nervousness, and hypertension. Caffeine is a diuretic and increases urination. It can decrease a person's ability to lose weight because it stimulates insulin secretion, which reduces blood sugar, which increases hunger. Caffeine can help to relieve headaches, so a number of over-the-counter and prescription pain relievers include it as an ingredient, usually with aspirin or another analgesic.