The Food and Drug Administration approved the first drug shown to prolong the lives of people with the skin cancer melanoma. The drug, Yervoy, is part of an emerging class of drugs called immunotherapy that uses the body's immune system to fight tumors. Because the approach isn't specific to the type of tumor, it may work for others, though this hasn't been proven in trials. But because it works by taking the brakes off the immune system, it can lead to other problems, like hepatitis and endocrine dysfunction. It will also be expensive. The makers of the drug, Bristol-Myers Squibb, says it will charge $120,000 for a full course of treatment, which consists of four infusions over a three-month period. The high price raises questions about drugs that prolong people's lives by only a few months: In the trial, patients treated with Yervoy lived a median of about 10 months, compared with 6.4 months for people in the control group. More than 20 percent of the people who took the drug lived at least two years.