>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)

[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)