Casey76 touched on the subject but to add my tuppenth :lol: .
Alcohol rarely interacts with medications, it does however interfere with the metabolism of the drugs, ie, the way they are broken down in the body, this is usually via an affect on the liver.
It can thus increase metabolism (break down) or decrease metabolism (break down). In short term meds this may not be a problem....however...if you take meds long term the effects can be cumulative meaning the amount of drug in your system increases day by day until side effects are noticed...this can be very dangerous. It's also worth noting that whilst alcohol may not interfere with the initial medication in question it may interfere with it's break down products which can be equally as dangerous, sometimes more so.
Speaking as someone who worked within oncology for 12 years I have to say that the advice of the doctor whilst appreciably sensible for short term meds, was totally inappropriate from an oncology point of view, or the view of long term medication.