An antiemetic is a drug that is effective against vomiting and nausea. Antiemetics are typically used to treat motion sickness and the side effects of opioid analgesics, general anaesthetics, and chemotherapy directed against cancer. They may be used for severe cases of gastroenteritis, especially if the patient is dehydrated. Antiemetics, though previously thought to cause birth defects, have been proven safe for use by pregnant women in the treatment of morning sickness and the more serious hyperemesis gravidarum. Bismuth subsalicylate works by protecting the stomach lining. Antihistamines appear to dull the inner ear’s ability to sense motion. They “block” messages to the part of the brain that controls nausea and vomiting. This is why they work best if you take them before you think you might have a problem with motion sickness.