Very dangerous. Especially with Xanax. And I'm not a "sky is falling!" hyperbolist. Xanax and 'done are one of the most common causes of fatal ODs. Search it up on Google and Opioph***.
I recently did a lot of research on this when my new PM doc put me on methadone for CP (30-40mg day in 3 doses). I've been taking Xanax for therapeutic needs for many years. Keep in mind that I never take Xanax recreationally. It doesn't do anything for me and I've always maintained on a low dose (.5-1.5mg tops. Usually .5mg). It's been great for controlling my anxiety disorders, panic attacks and insomnia, but I'm getting off track.
Adding alcohol to the mix increases the risk geometrically. I know from experience that one can drink 4 beers, take .5 mg Xanax and 30 - 40 mg methadone in a day and be fine (buzzed, not even close to nodding). HR: I am not recommending that. If you have to take your Xanax for medical reasons, just take it real easy.
Everyone reacts differently to drugs so I recommend starting really low with the Xanax. I started at zero and worked up with crumbs because I had been scared to death by most advice I'd gotten on other boards. I was also understandably concerned because I know of dozens of people who have OD'd on done and Xanax. It is a real danger.
You will get some very dire warnings from people. They're right. But if you're extremely careful and don't take your Xanax irresponsibly, you will be okay.
If you're one of the many people who have taken more benzos than you intended in a night, the combo is very dangerous. I've never awakened to find any Xanax missing. But a lot of people do. Just sayin'. Not something to take lightly.
And watch the alcohol. I can't recommend drinking on this combo at all.