Since Tomb Raider III Lara can get poisoned by certain enemies or traps. For example she will be poisoned when she is hit by poisonous darts coming out of the walls or being shot by the natives in Tomb Raider III, or being bitten by a cobra or stung by a poisonous scorpion.
You can see that she is poisoned by the Health Bar turning yellow. It will slowly start to run down till Lara eventually dies. She has to take a medipack or pick up a medi crystal (available in Tomb Raider III) to heal.
In The Last Revelation it also makes your view strange, when you have the effects turned on, meaning judging gaps etc. is harder.