I have received an anonymous letter accusing an employee of taking drugs, in and out of work, and I am suspicious: can I test him/her for drink and drugs?
If this is happening in your company right now you are right to take this information seriously especially if you are in an industry where this could lead to potentially dangerous situations
(operating machineries, driving, dealing with children and/or vulnerable adults etc.).
Selective testing can only really take place after an incident has taken place or when there is reasonable and quantifiable suspicion (with a policy in place that has been previously communicated).
You need to be mindful that this allegation could be malicious or simply a misunderstanding and, if you have no evidence, I would be wary of rushing in and start testing people.
It is more productive to establish a work culture where people feel that, if they have a problem, they can talk to someone openly whilst understanding their responsibilities to ensure their turn up for work in the right condition.
If you really wanted to introduce random testing, this could be done on health & safety grounds albeit you will need to consult with employees, clearly explain why and how these tests will take place and they will need to give their consent