I wonder if that's true though. I don't have a medicine background but I'm loosely following this development. I think the term drug resistance became popular with all these tuberculosis cases. What happened was that people got TB, they went to the doctor and got medicine. Then things got better and people felt healthy again so they stopped taking their medicine. BUT in reality they were supposed to take it for a longer period of time and then TB broke out again. And thus becoming drug resistant.

https://www.who.int/tb/areas-of-work/drug-resistant-tb/xdr-t...

Right. For resistance to evolve, some part of the treated population must survive. (You can't evolve resistance to atomic weapons dropped on your head.) Is PReP really effective enough to ensure that there are no survivors? Sure, maybe it is, used correctly at therapeutic doses.

What if it becomes common enough to get into the water at much lower doses? What if people split pills to reduce costs? ISTM that there are many potential avenues for evolving resistance.

HIV is sexually transmitted. It can’t survive in the water or the air. So the presence of anti HIV drugs in the environment shouldn’t have an impact.

It doesn’t really work like that because of the lifecycle of HIV.

If someone is using PrEP and is exposed to HIV, then either the drug works and they avoid infection (in which case there is obviously no impact on drug resistance) or they become HIV+. But in that case, they were either exposed to an already-resistant variant, or they were astonishingly unlucky and the drug failed to prevent infection - in which case, resistance is a moot point.

A person needs to develop an active HIV infection in order to communicate the disease to another. PrEP prevents this from happening in the first place.

Or the person was not taking PrEP as prescribed, either missing doses or taking it inconsistently. Once they're infected, if they keep taking just Truvada, then that strain will begin developing resistance, which they can pass on, potentially to others taking PrEP correctly. There have been I believe 6 cases of people being infected with resistant strains while in PrEP.

That's why it's important for people taking PrEP to be screened regularly, and put on a proper cocktail if they test positive.

We don't give antibiotics prophylactically, except in the circumstances where it is seen as warranted:

https://en.wikipedia.org/wiki/Antibiotic_prophylaxis

(I'm talking about the use in humans there)