This is a very hard question, since it depends so much on the interviewer/recruiter. But I think you should look at it like this.
You've made an app for a specific market - regardless of your own opinion of the subject. That market is illegal, but are moving towards being legal.
Marijuana is a sensible topic since some people have a very colored view on people who consume/sell/buy or even talk about marijuana.
You might come in a situation where your recruiter doesn't support that you've made an app for this market, but you might also come across another recruiter who is a supporter - it can go either way.
So here is what I feel is the right thing to do:
Threat this in a very professional matter, acknowledge that there is a medical market as well as an legal market other places in the world, and that you simple made an app for this market.
I Assume that this app might be available in various app markets, is that true? If so, I don't recommend lying about it, by re-skinning the app, since it might come back to you in the future.
Full disclosure can help you find a place to work, who acknowledge the way you think, and who you are. Which might be important to you, or might not. But if you tell this doing the interview, you don't have to worry about it afterwards, which you might do, if the app can be traced back to you somehow.
Either way, I feel that you should try to sell yourself for who you are, and the quality you posses in an interview.
So my advice is this:
Put it on your portfolio, but threat it in a professional matter. Talk numbers in terms of amounts of downloads, daily users and so on.