Well, in my opinion, cough medicine is a waste of money anyway and I'm pretty sure there is research to prove that too. And you have to be very careful not to overdose as cough medicines can also contain ingredients that are in pain relief meds that you may not notice.

ETA - not saying that everyone is so unintelligent not to be able to read a label - but I've seen it first hand with a parent who presented 3 conflicting over the counter medicines for her baby and didn't realise they shouldn't be used together.