Buying it from a stranger means that there's a good chance it was laced with something that may have reacted to the pain pill.
Another thing to consider is that a high THC concentration (which I assume hash oil may have) might cause strange reactions - for example, I had a bad reaction to marinol when my Dr prescribed it, but pot (from a known source) was no problem.
FYI - I've heard there are some dispensiaries(sp?) in Cal that will sell to patients with valid out of state cards.
I have found that medical mj is the only thing that has ever helped my ON, and it also works wonders for pain and nausea. Unfortunately, I no longer have access to it because I moved to a state that doesn't allow it. So now I have to use rx drugs that have much worse side effects - at least for me.