The drug sildenafil, better known as Viagra has been shown in mice to support the immune system's response to cancer cells. Mice with colon and breast tumors showed that the tumor size decreased two and threefold when treated with sildenafil compared with the mice that did not receive the drug. The researchers also believe that they have evidence that sildenafil is effective on the immune system because in mice engineered to lack an immune system the tumors were unaffected. Tissue samples taken from head and neck cancer and multiple myeloma patients showed similar results.

Viagra is a class of drugs used to treat impotence. It is known to stimulate nitric oxide which hinder the effects of a specialized cell that can divert the immune system away from the tumor. The researchers are trying to figure out how cancer cells can avoid detection by the immune system. T-cells that mount the attack on cancer cells are manipulated by the cancer cells and prevent them from being seen by the immune system. Viagra can unmask the cancer cells and let the immune system recognize and attack them.