My understanding of the rules in the US is that they're pretty much the same as in the UK. Occasionally there are reports of "medical recycling" but it's not common, and the "rules" are vague enough to make many who could engage in pass-on meds extremely reluctant to be involved.

Additonally, tamoxifen is one of the drugs recently reported as being in extremely short supply in the US, with the result that there was a "flood(?)" of counterfeiting of it, which would significantly increase the difficulty of proving safe provenance for anything coming from "outside normal channels" that you might consider passing on to another patient.

And of course there's the additional risk that anyone found simply possessing any prescription drug that doesn't have the name of a licensed dispenser and the name of the person who has it on the package would likely result in arrest and an "illicit drug possession" charge from our ever zealous police and DEA.

John