General healing skill. Used to heal small cuts, abrasions and other minor ills. In game terms, the use of first aid can heal more hit points over time than just rest.”— In-game description
Fallout and Fallout2
Using first aid too much can cause character strain, preventing further use until enough time has passed.
In Fallout Tactics multiplayer games first aid allows you the ability to get your almost-dead warriors back to full health in one or two turns. This implies that you take one doctor in your squad with more than mediocre healing skills. Moreover, the healing via first-aid-kits is more cost effective than relying on stimpaks. The downside is that you mustn't move your healer and the healee, otherwise the process will be terminated (which means that you've wasted Action Points, which you don't want). Especially useful in conjunction with high hitpoint characters like brutish-hulkish Deathclaws.
Using first aid too much can cause your character strain or become addicted to the drug or food.