Cyanogen

Unbeknownst to the asshole dying on the floor, as I’d rolled into the room I’d broken open a vial of oxalonitrile, also known as Cyanogen. A colorless NFPA Level 4 gas with an LC50 toxicity of 0.06 parts per million. It interrupts the electron transport chain inside your mitochondria, preventing your cells from producing energy. That’s a complicated way of saying it’s really nasty shit. The tiniest whiff will knock you unconscious instantly.
...

“Where on earth do you get that shite?” said Gow, as together we hog tied The Wraith’s hands and feet.

“High school chem lab,” I said.

“You can mail order it?”

“No,” I said. “You gotta make it. Breaking Bad sucked, but it wasn’t wrong about the chem teacher bit. You can make pretty much anything if you know what you’re doing. The base reaction is just methane and ammonia in oxygen with a platinum catalyst. Methane comes out of the taps and you can buy ammonia at IGA. That gets you hydrogen cyanide. React the HCN with sodium hydroxide, a chemistry class staple, and you got aqueous potassium cyanide. Then it’s a simple reaction with Copper(II) sulfate, another staple, and you got yourself some Cyanogen. I can get you more by the end of the week. With nail polish.”

...

“You inhaled a nearly lethal does of Cyanogen,” I said.

“What you’re experiencing now is the worst headache of your life along with severe nausea. Both of which will persist for the next three to four days unless you take these.” I pointed to a nearby chair which held a small glass vial of amyl nitrate and IV bags of sodium nitrite and thiosulfate, all of which I’d mixed in the high school chem lab. The Wraith tried lifting his head. The motion made him puke again. Gow doused him with a re-filled bucket.

“Whether I administer them or not is entirely up to you,” I said. “Without the antidote, your mitochondrial system will sustain permanent damage. Climbing a set of stairs will feel like scaling Everest for the rest of your relatively short life.”
-Sledge vs. The Labyrinth, pg. 61-65