RC is also used in relation to drugs where it is the abbreviation of Research Chemical.
Some of the (slightly) more known chemicals that come under this category are 2C-B 2C-T-7 and 5-MeO-DMT
They generally come under the chemical groups of either Phenethylamine or Tryptamine, and for some reason seem to always be full psychedelic drugs, whereas the given description "Research Chemicals literally refers to chemicals that are still being researched" (from erowid.org) implies that any new, non-psychedelic drugs should also fall within this category.
Due to the way the drugs laws worked until recently it was relatively easy to get new research chemicals as they had to be made illegal as each substance was discovered or invented. These laws have recently been changed in most countries, but certain places (including parts of America) still have vendors for these substances. What these new laws did was to make any chemical within a group illegal without each having to be added individually.