We get a casualty/fatality number for the Federation-Klingon War of 100 million, which would put it around twice the amount of dead for World War II, which given the interstellar nature of the conflict is arguably not that bad in the bigger scheme when each sides starships arguably have the ability to glass a planet with anti-matter based weapons.
On the other hand, since the war only lasts for like a year, that number would indicate that more than a few colonies were obliterated and may have been raining blood, given that starships of the era only carry crews in the hundreds, and I don't think either side lost thousands of ships in the war in that short of a time. So gods knows how many colonies along the Klingon border are just gone.
Also, I did like the touch that for all their bluster and warrior bullshit, the Klingons may have only moved the border 3-6 light-years. It's basically the equivalent of the (space) Battle of the Somme.
That would certainly fit with Tasha Yar preaching to Wesley about drug use in TNG lol.
I have a feeling it's something that's alien which M'Benga might have discovered during the war, perhaps a secret that only Chapel and M'Benga know about, and has some nasty side-effects they haven't expanded on just yet. M'Benga and Chapel seem very reluctant to use it. And if it was standard issue, and something known and issued by the Federation, you would think they would of broke it out when they were having issues with the Gorn last season
To me, it felt like the writers couldn't reason a really good way to get Chapel and M'Benga off that ship another way, and decided to lean into well they just fight their way off. We just gotta go with that the Klingons are stupid and no one decides: "Hey, why don't we just shoot them with a disruptor?"
Although, there was an option they could have gone with and didn't. The story kinda loses track of La'an and could of just as easily set up her coming to the rescue of Chapel and M'Benga, and all of them having to find a way off the ship together, instead of going with Super-Soldier Serum.