I have some background in pharmacology, particularly psychopharmacology and I can tell you that it's not even remotely true. There's so much studies out there... so, what kind of studies were you reading?
For example - Rapamycin/sirolimus, ephedrine, morphine, antibiotics (e.g. penicillins, cephalosporins), psilocin and psilocybin, are the first naturally occuring (in significant quantities) that came to my mind when thinking about some. Neither of these were studied only in any combination drugs (FDCs). And a lot of close derivatives of naturally occuring substances, like aspirin. This could be really a huge list (of both).
Was the claim I was responding to justified anyhow, though?
It's literally one query to https://scholar.google.com or any other search engine of choice, should I link to search results? Okay, so append ?q=[substance name] at the end of the URL. I don't mean to come off as rude, though. But it's really simple to do so, and it won't really serve any purpose to link to particular studies, either, as this could in turn add a potentially huge selection bias.
By the way, for more "obscure" in terms of scientific interest compounds see: beberine, theanine (aka l-theanine), vinpocetine. Particularly the last one. Same. These ones don't even have any synthetic counterparts which will share same or similar mechanism of action (there are other synthetic compounds targeting the same receptors, but they still represent very unique combinations of targets and also often have unique pharmacodynamics in relation to these targets).
For example - Rapamycin/sirolimus, ephedrine, morphine, antibiotics (e.g. penicillins, cephalosporins), psilocin and psilocybin, are the first naturally occuring (in significant quantities) that came to my mind when thinking about some. Neither of these were studied only in any combination drugs (FDCs). And a lot of close derivatives of naturally occuring substances, like aspirin. This could be really a huge list (of both).