The interviewers advise you of their pre-employment (pre-offer) drug screen and ask if you'll pass, so they can decide if they want to continue with you in the application process. Residency programs invest a lot of money in screening applicants, and they want to avoid sending you for a drug test if you will not pass.

As with any interview question, you want to focus on being completely honest in your response. As embarrassing as it may be, if you do not think you will pass the drug test, you need to inform the interviewers now. If passing the drug test will not be an issue, ensure the interviewer that you will pass the test because you do not use drugs.