Question: I've been unemployed and job searching the past few weeks. I have a medical marijuana card which I stupidly utilized during this break. Now that I have hit my final interview, I'm a little concerned. I've quit for a while but know it's still in my system.

I know excuses don't matter in the workplace, and a failed UA means you're not getting hired. My question: So I'm flying out of state for this last interview and they mentioned they'll take care of the hr stuff there. Is it possible to have an onsite drug test or will they most likely order one for me in my home state when I return?

Answer: 
I would count on it.

The answer though is it depends on the company you are applying to and the state where you are interviewing. 

My company screens all candidates for certain positions. And none for others. I know for a fact that one of my colleagues has all "real" candidates take drug tests. These people are dealing with multimillion dollar FX contracts and one mess up can be devastating. 

Also there are state laws that add some variance. Some states are more lax on drug testing while others make employers give you a conditional offer before they are allowed to test you. A quick google will give you the state drug info on the state you are going to. 

If this company was giving you a drug test I can't see it happening how you propose - when you get back. No way I have an out of town employee coming in and give him a job offer while he completes drug test back home at some lab I don't know. 

My advice - since you are out of town and since this company isn't in a huge rush. Stop. Think of a good excuse to delay the interview. Get tested at a local lab and don't go until you pass.
I'm going to take the opposite opinion of some others and tell you not to worry about it until it comes up. Go to the interview as planned, and don't bring it up at all. I would not expect them to surprise you with a urine test at the interview, but I wouldn't entirely rule it out either. I would more likely expect that they require it within the first week of work, or that you schedule one with a testing facility.

If at some point they tell you that they will be giving you a drug test, then be honest about it. Try something like this:

> 
> Actually, since medicinal marijuana is legal in my state, I have a medical marijuana card to help me treat some personal health issues. I know that it can be detectable up to XX days after consumption, so there is a slight chance that I would fail that portion of the test. I understand that marijuana is not legal in this state though, and I most certainly would commit to not using in the future. If there would be problems with my test coming back positive for marijuana right now, could we delay the test until a later date?
> 
> 
> 

You want to be as factual and informed as possible. Know how may days it is possible to be detected. Know the laws in both states. Explain your illness if you feel comfortable. Show that you are a responsible legal user, not just some "pothead".