Potions of Fortitude raise the constitution of the consumer to 18 for 15 turns. It's sold at Sorcerous Sundries and other stores in the game of Baldur's Gate. It may be found in containers all over the Sword Coast. In Baldur's Gate II it is sold at the Trademeet Smithy and at the Brynnlaw Store.
Once consumed, this potion will send a surge of energy to travel through the body raising the drinkers constitution to 18. This provides all bonuses of having a high constitution such as hit points for the duration of the effect, which is 15 turns.