Thalidomide, sold under the brand names Contergan and Thalomid among others, is an oral medication used to treat a number of cancers (e.g. multiple myeloma), graft-versus-host disease, and many skin disorders (e.g. complications of leprosy such as skin lesions). While thalidomide has been used in a number of HIV-associated conditions, such use is associated with increased levels of the virus.

Common side effects include sleepiness, rash, and dizziness. Severe side effects include tumor lysis syndrome, blood clots, and peripheral neuropathy. Use of thalidomide in pregnancy can cause fetal abnormalities such as phocomelia (malformation of the limbs). In males who are taking the medication, contraception is essential if a partner could become pregnant. It is an immunomodulatory medication and works by a number of mechanisms, including stimulating T cells and decreasing TNF-α production.

Thalidomide was first marketed in 1957 in West Germany, where it was available over the counter. When first released, thalidomide was promoted for anxiety, trouble sleeping, "tension", and morning sickness. While it was initially thought to be safe in pregnancy, concerns regarding birth defects arose, resulting in its removal from the market in Europe in 1961. The total number of infants affected by thalidomide use during pregnancy is estimated at 10,000, of whom about 40% died around the time of birth. Those who survived had limb, eye, urinary tract, and heart problems. Its initial entry into the US market was prevented by Frances Kelsey, a reviewer at the FDA. The birth defects caused by thalidomide led to the development of greater drug regulation and monitoring in many countries.

It was approved in the United States in 1998 for use as a treatment for cancer. It is on the World Health Organization's List of Essential Medicines. It is available as a generic medication.