Well, for one ... it's illegal to import drugs that aren't FDA approved. You can import a few months personal supply of a prescribed drug that is FDA-approved but doesn't just mean that the active-ingredient is FDA-approved. It has to be the same exact product, manufactured in the same FDA-approved facility with the same packaging/labeling/etc to be considered "FDA-approved". The most expensive FDA-approved drugs are sold at US prices globally, so there's no geographic arbitration. Then other non-FDA approved brands are sold at lower prices - importing these is a smuggling offense, though enforcement was pretty low (but now with CBP's upcoming budget increases, who knows if this will continue to be practical or ultra-risky).

More practically - HMG is a very difficult drug to assay for purity. It's too complex to interpret with qNMR, HPLC testing is also very hard to interpret. The testing that exists evidently either has a very high margin of error or involves lots of rats and dissections.

Even testing for hCG, while it can be done reliably with HPLC, results between different labs are not comparable because the primary assay is also to test bioactivity on rodents, so they're not normalized to the same standard.

The lack of any independent testing for HMG means that some of the more accessible international manufacturers don't actually test their own product. Combined with its high price, that all makes it a very common target for counterfeit.

Yes, the American pharmaceutical system absolutely has quality-control issues. 80% of our generic pharmaceuticals come from overseas production. The pentagon wanted to independently test the drugs it was purchasing for the VA, so it worked with a company named Valisure who determined that about 10% of the drugs had issues with contamination or a lack of the active ingredient[0]. The FDA responded by shutting down Valisure's third-party testing.

But even with the problems we have here in the USA, HMG is the one drug I would particularly not trust from gray-market supply chains. It's conjecture, but I wouldn't be surprised if the doctor said that because other patients had tried it and had poor results.

0: https://archive.ph/ubLmg