"For R&D use only" is intended to prevent the general public from using the chemical as a medicine.
It's quite common for medicine manufacturers to use AR grade chemicals in formulated products, but only after going through a major qualification and approval process involving the specific supplier.
The Analytical grades from chemical suppliers usually have to meet published specifications, such as ACS Reagent Chemicals, and the catalogue specification usually lists which standards are complied with. These are chemical criteria, not medical safety criteria.
In general, if the chemical has no detailed specification, it's very unlikely to meet AR grade, otherwise it would be sold as that grade ( more expensive, more profit ).
Most manufacturers catalogues and websites will detail the different grades. You choose the grade that is appropriate for the analytical technique you are using.
Note that most expensive grade may not be appropriate for all applications, or even be the most pure. For example, chromatography and spectroscopy grades of a salt may have more impurities than an AR grade, but the residual impurities present in the special grade don't absorb in the UV, or have trace metals that would compromise spectroscopic analysis.
