Can my doctor deny me pain medication because of marijuana use?

That is an interesting question. It is my suspicion that a doctor would be justified in doing so for two reasons: 1) cannabis remains a Schedule I narcotic under federal law, and 2) most authorities will defer to the medical judgment of the treating physician. Therefore even if you reside in a state allowing medical cannabis use, the doctor could still deny on the basis that it is an illegal substance (absent prescription, of course).

Your doctor can use any criteria he deems necessary to deny you treatment. Health Care is still performed as a business transaction -- and there are only very limited circumstances where a doctor MUST provide a service.

Likely the doctor is concerned with the drug abuse. (Marijuana) Using pain killers with other drugs could be a lethal combination... and he certainly doesn't want to be sued by your estate for malpractice.

A doctor would prescribe those drugs he feels is needed and would not be effected by other medication or drugs you take.
So yes, illegal drug use can effect your treatment plan