It's mentioned separately, but not giving any further reasons:
> A global data safety monitoring board will look at interim results at regular intervals and decide whether any member of the quartet has a clear effect, or whether one can be dropped because it clearly does not. Several other drugs, including the influenza drug favipiravir, produced by Japan’s Toyama Chemical, may be added to the trial.
I'm not finding the original statement/source from WHO, I'm sure they gives more reasons there around why some drugs are not included (yet)
Most likely, yes. It's not generally available outside of Japan (and China where it has also been tested) so not much point in adding it to a global trial.
more to do with the side effect. it causes birth defects in pretty alarming rate.
Japanese government only allows it to be used as a backup, Chinese licensed the design but the government only allows it to be used when all other avenues are exhausted.
That isn't the issue; it's that this drug is not available at all outside of those two countries. Side effects are the reason it isn't. The WHO is prioritizing drugs with worldwide availability.
Interesting stuff here. I'm glad to see that some of the conclusions I came up to over time turned out to be named and well-understood best practices. But it seems I have more work to do.