The word "overdose" implies that there is a common safe dosage and usage for the drug; therefore, the term is commonly only applied to drugs, not
poisons, though even poisons are harmless at a low enough dosage. Drug overdoses are sometimes caused intentionally to commit
parasuicide or as
self-harm, but many drug overdoses are accidental, the result of intentional or unintentional misuse of medication. Intentional misuse leading to overdose can include using prescribed or unprescribed drugs in excessive quantities in an attempt to produce
illicit drugs of unexpected purity, in large quantities, or after a period of drug
abstinence can also induce overdose.
Cocaine users who
inject intravenously can easily overdose accidentally, as the margin between a pleasurable drug sensation and an overdose is small.
 Unintentional misuse can include errors in dosage caused by failure to read or understand product labels. Accidental overdoses may also be the result of over-prescription, failure to recognize a drug's active ingredient, or unwitting ingestion by children.
 A common unintentional overdose in young children involves multi-vitamins containing iron.
Iron is a component of the
blood, used to transport
oxygen to living cells. When taken in small amounts, iron allows the body to replenish hemoglobin, but in large amounts it causes severe
pH imbalances in the body. If this overdose is not treated with
chelation therapy, it can lead to death or permanent
coma. The term 'overdose' is often misused as a descriptor for
adverse drug reactions or negative
drug interactions due to
mixing multiple drugs simultaneously.