A patient with tuberculosis develops bright orange-red urine and calls his physician in a panic because he is afraid he is bleeding into the urine. The patient has no other urinary tract symptoms. Which of the following medications is most likely to produce this side effect?

While not of major medical concern, urine color changes as a result of medications can be very distressing to patients. These changes are consequently woh learning so that you can warn patients when you prescribe the medicine. Rifampin is a safe drug that is used both for prophylaxis and for active tuberculosis therapy. In addition to discoloring urine, rifampin and its metabolites can discolor feces, saliva, sweat, and tears (and apparently can stain soft contact lenses). Ethambutol is an antituberculosis agent that is added if isoniazid-resistance is suspected and does not alter urine color. Isoniazid remains the principle antituberculosis agent and does not alter urine color. Pyridoxine is used principally in prophylaxis against tuberculosis and does not alter urine color.