I think phillip here is not referring to OTC painkillers. When I hear the word "painkiller" I think of narcotics, not asprin or ibuprofen. Why would you use narcotics for headaches? Or is your question more for their recreational use since you mentioned it would be "nice"? If I get a headache, I pop an ibuprofen like advil and it takes care of that. Nothing "nice" about it, just takes care of my headache. Narcotics DO hinder protein synthesis and for a lot of people it affects appetite as well. In addtion, it may also cause insomnia which further hinders your ability to grown due to insufficient sleep. If your looking to train seriously and grow some muscles, you have to decide:
party with painkillers and stay skinny or be healthy and grow. You can't do both bro.