This is not correct. When I held a clearance (which I don't anymore and won't again) the Department of Defense never made me take a drug test. I did, however, work for a publicly-traded government contractor that made me take one both times that they hired me as a matter of course. I worked for three other such defense contractors that were happy to have me without such a test.

I thought it was just a mild inconvenience until I developed a sinus infection and started taking pseudoephedrine and diphenhydramine two days before I was scheduled to take the test. Because these OTC drugs can cause false positives for methamphetamine and marijuana respectively (depending upon whether the test is reagent-based or GCMS) I had to quit taking them and suffer through two days of an upper respiratory infection to pass the test and placate some HR drone in Alabama.

I'm sure that there's a policy or exemption process for this situation but the same company also almost pushed my start date because their background verification service couldn't verify my past employment _at the same company_, i.e. their onboarding team was grossly incompetent. I decided not to chance it as I didn't want to go weeks without a paycheck while they sorted out a false positive. Needless to say I don't waste my time dealing with this class of employers any longer.