...poorly? The ICD-10 has been criticized quite a lot for where it splits categories; arcane problems get specific codes, while common issues are both fuzzily spread across buckets and conflated within overlarge buckets.
That said, I think the specific rationale is that X44 is meant to hold all types of low-frequency poisoning by drugs, and 'unknown drug' usually means "not one of the notable drugs that gets its own code and is tested for". This usually works alright. X45-X49 handle cases like "alcohol poisoning", "tainted food", "inhaled chlorine fumes", and "other/unspecified chemicals", so X44 is only representing drugs. And "poisoning" is separate from "adverse effects" (Y40-Y84), so an allergic reaction or a standard risk like bleeding from warfarin won't get mixed in here. Given that, X44 is just a reasonably narrow category to assign a group of T## codes to. Those go into enormous detail, and have secondary "unknown" categories to handle the case where you know the class of drug but not the specific drug. Totally unknown poisonings are restricted to X44/T50.9 and should be extremely rare.
But this was all designed by 1990; it looked reasonable to divide both X and T categories by "non-opioid analgesic", "anesthetics", and "narcotics". It wasn't until we started widely prescribing strong opioids for pain that "painkillers plus heroin" became a key grouping you'd want to research. That, plus the international nature of the system, also left us with "heroin", "opium", and "other opioids" as the only 3 T codes for this topic. The listed examples for 'other' are morphine and its prodrug codeine; as a result we've got fentanyl and morphine in the same category despite a 100x difference in strength.
Compounding that, the narcotic poisoning (X42/T40) grouping was basically doomed from the beginning through no fault of the designers. It tries to replicate the legal/treaty category of narcotics, which has no biological coherence; what on earth do cocaine and morphine overdose have in common? Why is "cannabis derivative poisoning" singled out in the 'narcotics' section of a document created when the only synthetic cannabinoid in use was a rare antiemetic no one abused?
tl;dr: It was supposed to be a low-frequency grouping with subcategories clarifying about specific drugs, but that fell apart for opioids.
That said, I think the specific rationale is that X44 is meant to hold all types of low-frequency poisoning by drugs, and 'unknown drug' usually means "not one of the notable drugs that gets its own code and is tested for". This usually works alright. X45-X49 handle cases like "alcohol poisoning", "tainted food", "inhaled chlorine fumes", and "other/unspecified chemicals", so X44 is only representing drugs. And "poisoning" is separate from "adverse effects" (Y40-Y84), so an allergic reaction or a standard risk like bleeding from warfarin won't get mixed in here. Given that, X44 is just a reasonably narrow category to assign a group of T## codes to. Those go into enormous detail, and have secondary "unknown" categories to handle the case where you know the class of drug but not the specific drug. Totally unknown poisonings are restricted to X44/T50.9 and should be extremely rare.
But this was all designed by 1990; it looked reasonable to divide both X and T categories by "non-opioid analgesic", "anesthetics", and "narcotics". It wasn't until we started widely prescribing strong opioids for pain that "painkillers plus heroin" became a key grouping you'd want to research. That, plus the international nature of the system, also left us with "heroin", "opium", and "other opioids" as the only 3 T codes for this topic. The listed examples for 'other' are morphine and its prodrug codeine; as a result we've got fentanyl and morphine in the same category despite a 100x difference in strength.
Compounding that, the narcotic poisoning (X42/T40) grouping was basically doomed from the beginning through no fault of the designers. It tries to replicate the legal/treaty category of narcotics, which has no biological coherence; what on earth do cocaine and morphine overdose have in common? Why is "cannabis derivative poisoning" singled out in the 'narcotics' section of a document created when the only synthetic cannabinoid in use was a rare antiemetic no one abused?
tl;dr: It was supposed to be a low-frequency grouping with subcategories clarifying about specific drugs, but that fell apart for opioids.