The distinction between drug abuse and drug misuse doesn’t reflect a difference in risk — abusing or misusing drugs can be dangerous or even fatal.
According to the FDA, drug abuse is the use of drugs without a prescription or for reasons unrelated to treatment. For example, even if a person had a valid prescription for an opioid, they would be abusing the drug if they purposefully took higher amounts than indicated or combined it with other substances.
Drug misuse, on the other hand, is what happens when a person is treating themselves but not according to the instructions on the package or from a doctor. For example, if you are still awake after taking a prescription sleeping pill so you take another when it’s only been one hour. If the instructions tell you to only take one every 8–12 hours, that’s drug misuse.