They try to work out which one killed them. If they can't, it should to my knowledge be filed as an unknown-substance drug death, although the toxicology report

They try to work out which one killed them. If they can't, it should to my knowledge be filed as an unknown-substance drug death, although the toxicology report would have the details. Forensic toxicology screens are pretty thorough, and benefit from being able to take invasive samples like heart blood, brain, liver, and even eye tissue samples. That provides a lot of flexibility to find any substance that's present and look at relative concentrations and metabolites. Toxicology is usually sufficient in combination with the coroner's report on proximate cause of death; even if multiple substances are found, it's only a problem if they're all able to kill by the relevant mechanism. In this example, someone with a high alcohol tolerance might die with a potentially-lethal level of alcohol in their blood, and also test positive for opioids. A tox screen should then be able to estimate that they had stopped drinking a few hours before (e.g. high acetaldehyde levels in blood and urine, no alcohol in stomach fluids), then freshly used opioids just before the time of death. This gets harder if you have drugs with additive effects (e.g. death by respiratory depression, testing positive for heroin and ketamine); the autopsy would list both, but I don't know how the stats are handled. And it seems like a nightmare in the case of drug interactions; I have no idea what happens if somebody dies of serotonin syndrome triggered by mixing alcohol with MAOIs.