Drug tolerance is your reaction to a drug after use over time. Tolerance is influenced by how much is taken and how often. Accidental overdose can happen as a result of higher tolerance, for example if a person takes higher doses trying to get the desired effect.
Tolerance to some drugs such as GHB can develop quickly meaning more is needed to get the desired effect but also meaning that you can experience withdrawal.
A lowered tolerance can happen if you haven’t used in a while so your tolerance to the drug may have changed. People who have stopped use and return to use are considered a high overdose risk. In these cases people need to remember to reduce amounts to see how their body reacts.