Lysergic acid diethylamide (LSD), also known as acid, is a hallucinogenic drug. Effects typically include altered thoughts, feelings, and awareness of one's surroundings. Many users see or hear things that do not exist. Dilated pupils, increased blood pressure, and increased body temperature are typical. Effects typically begin within half an hour and can last for up to 12 hours. It is used mainly as a recreational drug and for spiritual reasons.