You absolutely can patent existing drugs. There's a whole scummy pharma industry that takes existing drugs that are widely used off-label and patents that off-label use.

I think it’s worth addressing this with more nuance.

Companies can get a METHOD-OF-USE PATENT (MOU) on an old drug for a new INDICATION.

This gives them the exclusive right to LABEL AND MARKET that drug for that indication for a period of time.

I however, doctors can prescribe and pharmacists can substitute generics for the new indication, regardless of the MOU.

For a company to profit off an MOU, they strategically need to create a new FORMULATION. This is a new dose or delivery mechanism (extended release, topical, etc). A new formulation can be protected with conventional patents that go beyond an MOU.

With an MOU + patent on a new formulation, the company has a brand where they are the only ones allowed to make the new formulation and the only ones with a product approved to be marketed for the new indication.

Getting FDA approval for the new brand is not the main hurdle for the company. To get insurers to pay the premium they want over the cost of existing treatment options, they have to show it’s safer or more effective than those existing options. Otherwise insurance will block it.

In principle, this means that repurposing should only enable companies to profitably repurpose off patent off label applications if they can provide a real patient benefit.

Whether this is the best or most efficient way to promote this kind of innovation, or whether it works as well in practice as it would seem in theory, is a separate question.

Could you name some examples so that I can read more about this?

You don't patent a use. You're thinking trademark.