Can i take bactrim (sulfamethoxazole and trimethoprim) if i am allergic to penecillin?

Yes: From the standpoint of cross-allergy there is none. Two questions. What is the allergic reaction to pcn? (are you truly allergic). And then, since patients prone to true allergic reactions have a great chance of reacting to sulfonamides (like in bactrim), not because of cross-hypersensitivity, but because sulfa drugs have high frequency of allergic reactions. Discuss this with your doctor.