What is a poison that can be given to a comatose patient that requires another character to get an antidote?

Question

Basically, I need one main character to be comatose and the other one trying to save him. The antagonist has to give the main character a specific time period to jump through hoops or whatever to get the antidote.

I need something that either the killer injected before the accident that caused the coma (severe blood loss) or something he can administer afterwards.

And of course there needs to be an antidote as well.
Thanks for your help!

Edit: Adding some more information

The poison could definitely cause the coma. And for sure X has to wake up. As for the time frame I would say from the injection of poison it could happen right before the accident. The accident and the other stuff takes about an hour for him to reach the hospital. As for the antidote it's open atm it could be anywhere from 72 hours to seven days. The protagonist needs time to play the killer's game. And thanks for the response you guys!

Answer 1

The problem with poisons is they start damaging the organism as soon as they enter and continue doing so until completely neutralized - or, if the damage is made through a biological process, until the victim dies. So, even if the victim is going to die in 72 hours he can be doomed right after the first ten minutes of the poisoning, and there's no sense in looking for antidotes after that. Even when an antidote can work at any stage before death, the longer it takes you to provide the antidote, the more serious the sequels of the poisoning. Your character could arrive after 71 hours 59 minutes and administer the cure to save the victim, and he would live but as a very crippled, frail person for the rest of his/her (surely heavily shortened) life.

The way you have written your question makes me think you want something like a bomb, which can be defused in the last second preventing any kind of damage to occur. Simply, poisons don't work like that. You'll have to resort to nanodevices injected in the body which can be deactivated through some kind of futuristic nano-antidote, or maybe look for diseases instead of poisons. Diseases can be asymptotic for a few hours or days until they start damaging the organism, so it may work much more like you're looking for.

Answer 2

You can use pretty much any slow poison or venom

Once in a coma, the antagonist can simply inject the person and tell the character that their friend will die in X hours / days. Now you just need to work out how much of what type of poison will take that long to kill (and has an antidote).

The bite of a black mamba can potentially cause collapse in humans within 45 minutes, or less. Without effective antivenom therapy, death typically occurs in 7–15 hours.

Deaths from Black Mamba bites have been known to occur within 20 minutes with higher doses, but the average dose from a bite is 100-120 mg. Less venom will make this last longer, but making the dose too small will not guaruntee death.

There is an antivenom for black mamba bites.

Of course, you could use any other venom as long as you can find out how long it takes to kill and whether it has an effective antivenom.

Answer 3

Your main problem is that if you survive a poison for 36 hours, you'll likely survive. The damages can be very serious (even to the brain) but you'll be "alive".

Longest acting poisons that I know are botulism and some mushrooms with amatoxins (24-48 hours). Ricine may kill up to 72 hours.

Polonium-210 is very slow acting, but there is no antidote.