Probably because it's an opiate, like codeine, which is great for suppressing coughs.


Tramadol suppresses the cough reflex by binding to the mu-opioid receptor binding sites {10}. Due to the high affinity binding of the M1 metabolite to the mu receptor, the metabolite has been found to have more cough suppressant activity than the parent compound {10}. -http://www.drugs.com/mmx/tramadol-hydrochloride.html