First time hearing of Tanax. I did a little digging on the subject, apparently it's a formula containing three different substances, (embutramide, mebenzonium iodide and tetracaine hydrochloride). Embutramide is chemically similar to GHB and I would think that would serve as a sedative. Mebenzonium is a muscle relaxer/paralytic. The tetracaine is just for making the injection painless.
It reminds me of a critique of MAiD protocols I read where they were concerned that the muscle relaxers mask the body's reaction to hypoxia theorizing people feel like their drowning to death but are paralyzed so it looks like they're not suffering.
This article offers a comparison with phenobarbital in a pet euthanistic study which found it suboptimal. In some places I read that it's not used much anymore. I wonder if your vet just has back stock sitting around.
