It also looks different depending on the game. In Serious Sam 1, Serious Sam HD, Serious Sam: Next Encounter and Serious Sam: Double D, it is a floating white pill. In Serious Sam 2, it is a floating red and white semi-transparent set of pills. In Serious Sam 3: BFE, it is a bottle of pills.
The extra small health is one of only two health items that can boost your health beyond the usual maximum of 100, the other being the Super Health.
In Serious Sam 1, Extra Small Health is often used as a trap to spawn in extra enemies. Therefore, the player should avoid picking them up unless they have enough health/armor/ammo to deal with extra enemies that may spawn in.
In addition, chains of Extra Small Health appearing one after another when one is picked up is often a trap. After certain amount of Extra Small Health is picked up, an enemy will suddenly appear in front of the player.
In Serious Sam 3: BFE, the Extra Small Health is the most common healing item in the first two or so levels. They become less common later in the game.
In Serious Sam 2, Extra Small Health are often bundled up in groups of four or so to give the player a decent amount of health if all of them are picked up at once.