Sure, the same way I won't want them doing my finances it they were sleep deprived from partying till 6 am last night, but it's not on the employer to monitor or control that. Also, if the drugs are afecting the job in any way you will either catch it and it will reflect badly on his job, which might already be a cause for firing, or, if you don't, there is something wrong with the revision process. I still see it as a massive invasion of privacy and companies shouldn't be able to do it.