It's a tough balancing act between protecting your employees from anonymous tipsters with personal axes to grind against them while also protecting the Company, co-workers, and the public from the potential mayhem caused by a drug abusing employee. I hope you have a drug free workplace policy and that you address testing in that or a related policy.
If the Anonymous Tipster reports off duty drug use AND the allegedly drug abusing individual operates dangerous or potentially dangerous equipment, operates a motor vehicle, handles cash, or otherwise does anything for which drug abuse may pose a serious exposure to the Company or any person, then you need to send them out for a test. Document your reasons thoroughly, label the cause "reasonable suspicion", and simply tell the employee that you have a report of off-duty drug use and, because of their duties, it's cause for concern and you need to send them out for drug screening. Make sure they understand it's not a personal afront: the nature of their duties requires the Company to be highly sensitive to the potential problems caused by drug abusing employees.
Normally, when I get an anonymous tip, I get as much information as I can about the alleged drug use and have a frank talk with the employee of interest. If their denial is convincing and their potential drug use does not pose a serious risk then I document the episode and put it in an incidence file. Eventually, if there's enough smoke in the file, I'll be concerned about fire and send them out for a test. The documentation becomes a bullet for use later in a reasonable suspicion argument if that becomes necessary. If the person fails to deny ("what I do on my own time is none of your business") or denies unconvincingly, then I send them for a test.
I normally re-assign people in risk-posing positions to less-risky positions or suspend them with pay pending the outcome of the test.