You may be referring to capsaicin cream. This is a topical agent that acts as a counterirritant. The nervous system pays attention to the new messages of skin irritation. This may override messages to the nervous system from the primary (main) lesion or problem.
People with pain, numbness or tingling, or burning from shingles, amputations, and chronic pain may find a benefit from this treatment. Studies of patients using capsaicin compared to a control group (no treatment) or a placebo group (cream without capsaicin) show positive results for the capsaicin.
Some people may have an allergic skin reaction to capsaicin. Before spreading it over a large area, apply it to a small test site. You may want to put a dime-size amount of cream somewhere near your painful knee. If there is no problem after 24 hours, you can try it on a larger area around the knee.
There may be other more effective medical treatments available for your problem. If you have not had a physician examine you to diagnose the cause of your knee pain, this may be a good time to seek medical help first. Ask your doctor if capsaicin might be a good treatment method for your particular knee problem.