Gary's Mod Posioned Weapon (Not TTT for sandbox or DarkRP)

[Lex Umbram]

Hello, so I was wonder if someone could shed the light on how to make a poisoned weapon like a dart gun for Darkrp or sandbox. Orginonaly I had edited m9k nerve gas to be a gun and not have the smoke effect but when you shoot someone it would do 1 damage and then 1 damage for 90 seconds unless you Got healed. So if you had armor or started at 100 hp you would be left at 1 hp. But they changed it and made it so this is no longer possible sadly. Does any one know a new way or have a addon that could be used? Thanks.

What I trying to do:
Make a weapon that does one damage on a player it was aimed at.
Then over 90 seconds brings them down to 1hp or removes 1 health point every second for 90 seconds.
Would stop doing when player: 1. Dies, 2. Gets healed.