Thalidomide is a racemic molecule and the original drug was made and sold as a mixture of the 2 forms. The S form of thalidomide wasn’t therapeutically active like the R form, but was also the cause of the birth defects. Today, thalidomide is used to treat cancer and is a type of growth blocker for tumours. It interferes with chemicals that cells use to signal growth. It also stimulates some of the immune system cells to attack the cancer cells. Thalidomide stops tumours making their own blood vessels, which they need in order to grow and get the nutrients necessary for cell division.