Acute pain is often felt soon after injury, and is of short duration. Most people who have surgery have moderate or severe pain afterwards. Painkillers (analgesics) are tested in people with pain, often following the removal of wisdom teeth. Study participants have to have at least moderate pain levels and the pain is usually treated with painkillers taken by mouth. This overview is useful mainly for acute pain lasting only a few days or weeks, and not for chronic pain lasting for many months. For this overview we have not included information from reviews on migraine, tension headache, or period pain.