If something is 'research use only', what does that actually mean legally?

The phrase is on everything and I suspect it means less than it appears to. Does it have a legal definition or is it a formula people print?

Mostly the second, and it is doing a specific job.

"Research use only" or "not for human consumption" is not a licence, a category, or an approval of any kind. It does not describe the substance. It describes what the seller is asserting about the transaction, and its purpose is to move the compound out of the regulatory framework that governs medicines and into a much lighter one that governs chemicals.

Notice which direction the protection runs. The label is not there to inform you. It is there so that when somebody injects the contents, the seller can point at a sentence.

What it is genuinely telling you, if you read it straight: no medicines regulator has assessed this for safety, efficacy or manufacturing quality, and nobody is claiming they have.

So the label is closer to a disclaimer of liability than a description of the goods.

That is exactly what it is. And the manufacturing point is the underemphasised half.

Approval is not only "does this drug work". It carries good manufacturing practice with it: batch records, validated processes, inspections, a regulator entitled to walk in. A research-use chemical carries none of that, and there is no obligation on anyone to have made two batches the same way.

That is a separate question from whether the molecule does anything, and it applies even to compounds where the molecule is well characterised.

Which is why the "but it is the same molecule as the approved drug" argument does less work than people want.

It might be the same molecule. It is not the same quality system, and the quality system is a large fraction of what the approval actually was.

This is the thread I want to point people at when they say a research-grade version is "the same thing, cheaper". It might be the same thing. There is no mechanism by which you would find out.