Smoking substantially increases risks for several diseases, including lung cancer. Quitting smoking can be very difficult. To help smokers quit, doctors routinely recommend nicotine replacement therapy. There are several different forms of nicotine replacement therapy, including chewing gums, nasal sprays, and patches placed on the skin (transdermal patches). Even when people use these therapies, 70% to 80% of them return to smoking within 6 to 12 months. Researchers wonder whether some people might respond better to certain forms of nicotine treatment and achieve higher quit rates.