Poor Americans can't afford doxycycline, which used to be so cheap that it was easy for them to get.

What has changed? I think it is a reduction in the number of suppliers. If so, this is yet another result of allowing businesses to merge when it ought to be banned.

The market is a useful mechanism, but it is subject to instabilities. One of them is the tendency for all the producers of a certain good to merge. Without energetic government regulation, the market will go all wrong.