It can be fatal, of course, under the incorrect use, as with either substance on its own. There are some very legitimate uses for benzos during MMT: Sleep, anxiety, ataxia, etc. Naturally, each person’s doctor would make the proper decision based on their circumstances. The clown heroin addicts on MMT who are just looking for a high will quite likely face the silent death of respiratory failure in their sleep.
The other thing the junkies like to toss in the blender is anti-histamine. Apparently this combinations mimics and even exceeds a heroin high. It also invites the grim reaper.
I don’t understand why people do this.