Why is Matthew Perry's death listed as an "overdose" when toxicology tests are clear?

If there is no evidence of drug use or inebriation after a toxicology test, why is “overdose” listed as the cause of death?

The following excerpt from an article from the Daily Mirror gave me pause. Testing for meth or fentanyl is straightforward and can be detected in many ways using readily available tests. Our question is, if there is no evidence of drug use or inebriation after a toxicology test, why is “overdose” listed as the cause of death?

Initial toxicology tests did…