i've never seen a work app that asks if you smoke or not...
that aside, i wouldn't answer the question honestly. if they somehow found out, and i can't think of any job outside the government that would be able to access medical records like that, i'd simply tell them that the script was filled, but the drugs were never taken. i've been arrested for possession and still don't include that on job apps since i have a special probation (7411, for those of you in Michigan familiar with the legal system), so it doesn't show up on my criminal record unless i violate my probation- obviously key for thorough background checks.
the way i see it, marijuana shouldn't be illegal, so the arrest for possession should never have happened. it cost me a ton of money, but it won't be on my record, so it didn't happen as far as any future employer needs to be concerned. as for the usage issue, since marijuana use is so highly stereotyped in such a negative way, i would not admit to future employers that i shhhmoke da reefer. my recreational use doesn't affect my performance at work, so it's none of their damn business.