Not only that, but ingesting large amounts of live yeast can (does)cause explosive gastrointestinal results. A problem familiar to any home brewer that has consumed especially yeasty young beer.

If you get yeast in your intestines and you're taking antibiotics, you can get what's called Auto-Brewery Syndrome. The yeast in your intestines starts to ferment sugars and you get drunk all the time.

How do antibiotics cause this? Do they kill off some of the intestinal flora that would normally digest the sugars?

Funny that you mention that. I sit 1 desk over from a doctor who brought that up as I was writing my earlier comment. We all had a good laugh.

Why is taking antibiotics relevant to this effect?

Kill off competitive gut flora, making ecological room.

From what I've read ADH is something yeast has evolved with. It's not added to yeast.