I once had a concern about a co-worker, and that maybe she was using a stimulant at work. Her behaviour was becoming highly erratic, and one shift I worked with her, she was totally emotionally labile. I'll call her 'Julia', not her real name. She was acting like someone on meth, frankly. Also, another nurse, I'll call 'Anne' had vented to me at shift change regarding this Julia's erratic behaviour. I called Anne the next day told her that perhaps this gal was resorting to some drugs to help her cope. She had been working a lot of extra shifts, and everyone was noticing her volitile behaviour, which was escalating.
Anne and I who agreed that this was a possibilty and made a pact to not discuss this with another soul, and I said that I would approach Julia's manager, and Anne would back me up. I did this in all charity, because I had always liked Julia, and was truly worried about her. I discussed it at length with Julia's manager, telling her that Anne had said that she could call her, that I wasn't alone in my observations, and stressing that I was never going to discuss this with another soul, and neither was Anne.
I never discussed it with another soul, even my closest confidante at work. I'm sure that Julia was talked to because she cut back on her crazy hours, and made a point of approaching different co-workers, including me, that she had been having health problems and working too much, and had cut back. I don't know if she was using anything or not for certain, but it was a big reality check for her I think, and I don't have any regrets for having taken this action. It is a favor to all parties involved, but must be done with the utmost discretion.