Here is the deal. The sign at your clinic is true, not only for methadone levels tho, for many opiates. This is how it works.
Alcohol is a drug that is considered a CYP450 inducer. If you aint sure WTF that is just keep reading i will explain it to you. I am a 9th grade drop out Ghetto Education Diploma reciever, so if i can understand this shit, you can.
Yall know about potentiation, right? When for example you take some painkillers with certain other things like grapefruit juice, cimetidine (tagamet), etc. they work in your liver as "inhibitors" of the cytochrome P450 enzyme. That means that they slow the metabolizing of it and end up causing increased levels of the drug in your system with a longer duration.
the opposite is a cyp450 inducer. It causes the enzyme to speed up its action like the litle pac man guy on meth, chomping thru the little gum balls way too fast. the inducer causes cyp450 to increase its production of the enzyme protein that is responsible for metabolizing and processing the drug. so, instead of slowin it down and gettin the most out of it like in enzyme inhibition, in this case its the opposite. When this happens, it burns thru the drug way faster than it normally would have, breaking it down into its metabolites before it gets to you, weakening the drug. this causes abnormally low levels of the drug in your blood becuz its gettin processed into its metabolites too fast, not giving the drug enough chance to naturally work its way thru your system. the decreased levels in your blood serum due to this can cause withdrawal or make it come on sooner than if the cyp450 inducer had not been combined with the drug.
Alcohol is a cyp450 inducer and that particular enzyme is one that is very involved in opiate metabolization. so it aint something you want to combine with opiates if you want to get the most out of them. It will make them leave out your system faster than they normally would, its like putting the fast forward on them.
I hope that explains it for you and aint too boring. Its a common thing to hear people say alcohol makes opiates leave your system quicker, but not many people know about why that is so i am glad to explain it to you.