Dr. Zaid Fadul -- the Medical Director at BetterU and an expert in addiction medicine -- tells TMZ ... the L.A. County Coroner Office's finding last week that Perry died from the acute effects of ketamine doesn't really explain how he died ... at least in his professional opinion.

Yes, ketamine happened to be in Matt's system at the time of his death -- and might have been the reason he dozed off in his hot tub ... but obviously, the "Friends" actor drowned.

Dr. Fadul's point here is ... any kind of drug that spurs sedation would've likely had the same effect, and he feels like blaming MP's tragic passing on ketamine alone -- without this context -- isn't fair, and that might deter people from seeking it out as a viable option.

Now, on the issue of whether Perry had used ketamine the day he died or not -- Dr. Fadul has a theory on that as well. According to him, Matt was likely experienced with ketamine and was taking ever-increasing doses, which might've left some residual.

He adds, that residual amount could've been super high, if Matt had a high tolerance to the drug and was taking high doses. He also believes Perry wasn't only taking ketamine through an IV.

Remember, the coroner's report on Perry's cause of death noted he'd been involved in ketamine infusion therapy in the months leading up to this -- but pointed out he'd been weaning off of it thanks to a new doctor who found he didn't need as much these days.

Ketamine is often used as a treatment for depression and anxiety, which is why Matt had been taking it ... but because his ketamine levels were so elevated when he died, it's unclear Matt was taking it therapeutically or recreationally.

The implication from the report seems to be pointing to the latter. Dr. Fadul, though, isn't so sure that's accurate.