I think there's an issue of burden of proof at play. If these microplastics were a drug, they would need to go through an expensive and onerous vetting process that consists of both safety testing and efficacy testing for a particular indication for it to be legal to sell someone if and only if a doctor specifically prescribed it for that person. Even if you believe, as I do, that efficacy testing shouldn't stop people from voluntarily accessing potential cures, safety testing doesn't strike me as unreasonable
These microplastics are crossing the blood-brain barrier in a significant portion of the population without most people even intentionally ingesting them. Hell, it's hard to avoid if you try. Our society slathers this stuff over everything. In my lifetime I've seen a giant swath of packaging for food go to plastic from some other material, presumably to cut costs. I recently bought a new oven and I guess they put those stupid plastic phone screen covers over the glass panes on both sides of the oven door. Do they just ship all glass with this stuff applied to it now?
The burden of proof to ensure these things are safe should be on the companies selling them. The companies that make these substances already have vast research arms for designing the polymers, and if they were held to even the FDA's standards for drug safety we would have a lot more studies about this stuff. These companies are also heavily subsidized by governments, which might even be part of why downward pressure on price seems to make everyone go to plastics. Like why exactly is it cheaper, even if you don't bake in the entire R&D budget of inventing them, to use novel polymers made as byproducts of petroleum than it is to make glass or cardboard packaging?
We should, in the same way that we do for pharmaceuticals, hold these orgs to a standard of demonstrating that this bioaccumulation doesn't cause some kind of harm, because the alternative is that some researcher needs to come up with a specific claim of harm, fund a study despite there being significant pressure from these orgs to never do such studies, and certainly never publish if the results might credibly make them look bad, and somehow translate that into making bespoke positive policy prescriptions that would likely be fought tooth and nail by same. That expectation is not a sane default