It happens. I have been told by perpetrators that they have 'doctored' the food of particularly unpopular pilots, that steaks have been wiped around the toilet, and substances placed within their food. They have sworn blind to me that it happens, and some said they have done it themselves. It appalled me that these idiots have done it to their own pilots. I know on a few occasions, I was quite suspiciously and sharply upset inflight. Some tricks have been used without doubt. There is nothing you can do about it, nothing you can prove, but I absolutely know it has happened. From the horses mouth. And I believe it.