The present invention relates to methods for decreasing cellular DNA repair in a target patient; decreasing cellular NAD.sup.+ biosynthesis in a target patient; increasing efficiency of radiation therapy in a target patient; modulating nicotinamide phosphoribosyl transferase activity in a patient; or sensitizing a patient to a DNA damaging therapy. The invention relates to methods for treating a patient who received a toxic dose of an nicotinamide phosphoribosyl transferase inhibitor. The invention also relates to pharmaceutical compositions comprising a physiologically acceptable carrier; an effective amount of a NMPRT inhibitor; and nicotinic acid. The invention also relates to methods for treating a patient diagnosed with or suspected to have a cancer deficient in nicotinic acid pathway.