 [ Symptoms ] is it a tension headache if your headache never goes away even when you take tyenol [ Disease ] this is a very good question to my knowledge there is no serious adverse reaction associated with taking famotidine with an antibiotic however taking famotidine with an anti fungal oral medication such as the ones with names ending in azole can have serious side effects taking certain antacid or anti gerd medications like famotidine can reduce the effectiveness of other medications when taken together thus it is a good idea to wait 1 2 hours before or after taking other medications before you take a product like famotidine because you have finished your antibiotics good for you  the real question is do you still have symptoms of your illness if not then you did no harm by combining the two medications if you still feel ill then i would suggest you go back to your primary care provider explain the situation and get evaluated to see if you need another course of antibiotics in all likelihood everything worked out fine at least that is my hope for you