>That's the effect of NHC and of molnupiravir. What about the bad side? Well, auxh nucleosides can also be taken up by many other enzymes, including those that handle our own nucleic acids, so some of them are mutagenic. Indeed, that's how NHC was first characterized, as a mutagen in bacteria. They also tend to be cytotoxic, via a number of mechanisms, and nucleoside drug candidates are notorious for wiping out in human trials due to toxicity in the liver, kidneys, and other organs. That was a feature of the 2012 scramble in the hepatitis C area, where Bristol-Myers Squibb paid 2.5 billion for a nucleoside addition to its proposed therapeutic cocktail, only to see it all demolished within a few months when it turned out to have severe problems in human trials. From this story and others you can also conclude that you don't necessarily get a good reading on this stuff in animal trials - another feature of Fun With Nucleosides.[0](Derek Lowe)
There are quite a few nucleoside analogues that have been used for HIV treatment: AZT, Zerit, FTC, Videx, Abacavir, 3TC, Hivid. There may be others I'm missing.
Of these, only Abacavir and FTC (as part of Truvada) are still used, the others have fallen out of use or are no longer even manufactured due to their bad side effect profiles and propensity for severe adverse reactions. 'Nuke-sparing' regimen planning specifically would combine other classes of drugs to avoid this class due to side effect issues.
I think it's great to have this new treatment option but this class of medications is well known for its side effects. Avoiding vaccines as 'unproven' and then going for this in particular I would consider eye rollingly wrong headed.
[0] https://www.science.org/content/blog-post/molnupiravir-last-...
(edit: this is just a note I found, does not solve why/why not in my mind)