When you meet the powder ganger named Joe Cobb follow him until he stops.

Then talk to him, he will say something about Ringo and he will ask you to kill him.

Go and kill Ringo and then go back to Cobb. Tell him you killed Ringo.

Then he will tell you to get stuff for the Invasion of Goodsprings.

Kill Cobb and go to the doctor, tell him a lie and convince him to give you supplies.

You will not only get Stimpax, you will get xp.

You can do this as much as you want but you must have the medicine perk to do this.