As people become more tolerant to opiates
the more they take, long term users can take very large doses of drugs
and have very little effect, whereas the same dosage would cause death in an opioid
-naive patient. For example, a kitty I know takes over 1000 mg of oxycontin daily. That would knock out - and possibly kill - the average kitty. For that reason, TIs and LD50s are likely not available. LD50s are not only used for lab animals - they are discussed as well when talking about people - for example, you'll often read in reports that there is only a very narrow window between the LD50
and TI for acetaminophen
(which, in non-technical speak means that there is a very small window between the dosage of acetaminophen that gives pain relief (the therapeutic index - TI) and the dosage that kills 50% of people (the lethal dose for 50% of people = LD50). That's why there are so many warnings about Tylenol (acetaminophen) - because you can take 2 extra strength tablets 4x daily for pain relief, but double that and you're risking liver failure.