Deliver a sample of jet to Dr. Troy.: when the Chosen One manufactures the cure, Dr. Troy gives them two vials of the antidote, one of which is passed on to Doc Johnson in Redding to cure the miners of their jet addiction. The player is free to keep the second vial for themselves. The jet antidote is unique in that it can be used an infinite number of times without being consumed, effectively allowing the player to use jet anytime they want without fear of addiction. Players who use the unofficial patch, however, do not enjoy this bug: you must continue to buy more in San Francisco.
It is offered for sale by different merchants in San Francisco, but only after the Chosen One discovered the cure.
The jet antidote can also be found for sale by the gun merchant Cal on the PMV Valdez, San Francisco, who curiously carries almost no guns but loads of drugs. Vials of jet antidote are replenished if you buy them.