Lenalidomide capsules are a prescription medication used to treat a variety of different blood disorders, including multiple myeloma, myelodysplastic syndrome, and mantle cell lymphoma. The capsules contain the active ingredient lenalidomide, which is a derivative of thalidomide. Lenalidomide works by blocking the action of certain proteins that help certain cancer cells survive, which can cause them to die. It can also reduce the number of cancer cells and help reduce the size of tumors. Lenalidomide capsules can be taken orally or by injection and are usually taken once daily. They can cause side effects such as nausea, diarrhea, and headaches.