Tolerance.

For millennia, humans have sought to counteract the effects of harmful substances by taking small amounts of the substance and then gradually increasing its levels. This practice is called mithridatism, named for Mithridates VI, the former ruler of Pontus who, after his defeat by the Romans in 63 bce, tried to commit suicide by taking poison. However, because he had built up immunity to diverse poisons by years of incremental exposure to them, the poisons did not end his life and he had to resort to demanding he be killed with a sword. Mithridatism can be effective if the substance does not pass through the body too quickly or accumulate to dangerous levels; it works because tolerance to the drug has been established.