Hey Victoria,
Codeine is indeed a painkiller of the opiate variety...which means it's bound to slow you down! Most of the opiate drugs have the side effect on constipation. Actually, one of codeine's indications is for treatment of IBS and diarrhea (it's most commonly used for halting a racking cough). But take it with care, as it IS an opiate (albeit not a particularly heavy-duty one; but you can develop a tolerance to it).