The First Aid Spray is an Umbrella-produced health product which is designed to give immediate relief for injuries large or small. It's not designed to cure poison. It's unclear whether the Green Herb is used as it's base, but it's likely.
It has been marketed under various names over the years, including Cinatiropa, Life Safe, Medicinal Spray, and First Aid.
It's produced in a plain metal aerosol can with a white and green label, and is designed as a one-shot product to be sprayed over injured parts of the body. It appears the mechanism ensures that the entire can is exhausted from one press of the nozzle.
First Aid Sprays in most games heal the players health to 100%, although in the earlier titles the players Rank will drop by 1 each time a First Aid Spray is used. Due to the fact that this penalty existed, a lot of players assume this is still the case with the more recent games. That is false! Use them at your leisure apart from in Resident Evil Zero, Resident Evil, Resident Evil 2, Resident Evil 3: Nemesis and Resident Evil Code: Veronica / X.
In Resident Evil Outbreak and Outbreak File #2, the First Aid Spray heals 3000 HP, which in most cases will cover all characters' health.
Also, in most games the First Aid Spray is an extra item given to the player in easier modes.