I had an allergic reaction to bactrim (sulfamethoxazole and trimethoprim). What can I take for hives?

Antihistamine. Stop bactrim. You may want to take Benadryl (diphenhydramine) for your hives. Other medication that you can take if Benadryl (diphenhydramine) is not enough - include- other antihistamine like ranitidine, or claritin, zyrtec, Allegra etc. If otc medication does not help or symptoms get worse- see your md or urgent care.