Hang on, you are going to penalise someone for taking time off to get clean... presumably on the basis that it's better they come to work hiding the fact that they are high, than that they get help?
Every hire is full of risks. Nobody knows whether they'll get into a car crash, get cancer, or get shot, or become a substance abuser. Whether they admit their medical history or not. That's because they are human beings. Humans are inherently risky. If you want to eliminate risk, then hiring humans is not for you.
All you do by punishing people who are honest about their history, is to encourage even more people to lie in interviews. Those people are only the tip of the iceberg, and a significant number of people companies hire already have such history but they just keep their mouths shut.
This comment makes me sad. It seems illegal biases in hiring and management practices are rampant. Worker protections seem to be a joke when they're blatantly ignored.
Like the sibling comment, I find this very sad.
Perform drug tests if you're worried.
Otherwise it's none of your business.
This question applies to all employees. You don't, because it's none of your business if it doesn't affect their work performance. It is entirely possible, and probably likely that some your current employees use drugs. And almost 100% sure that some of them use alcohol, which is stronger and more problematic than many illicit substances. Many companies even throw parties where they give out free booze to the employees! How's that for a double standard?
People have always used drugs, and always will. Pretending to care about substance abuse only when you come across someone who openly admits their history is insane. You are treating the honest people worse than the people who hide their problems from you.
That's skirting uncomfortably close to discriminating based on medical history or a disability, which may be illegal depending on where the employer is located. Cancer survivors have a much higher chance of recurrence than others do of developing cancer for the first time, but most would probably balk at denying a job to someone whose cancer was in remission because there's a higher chance they'd need extended medical leave in the future.