The term watched chemical refers to a substance which is watched by the DEA (in the USA). This means that the chemical is perfectly legal to buy, however all sales of the chemical must be reported to the DEA.

The chemicals are usually precursor chemicals for controlled substances, like GHB, LSD, MDMA, etc. Normally these chemicals are not controlled themselves, and/or have some use in industry. An example is GBL, the main precursor for GHB has been used as a solvent to clean printed circuit boards. (interestingly, unlike others GBL is psychoactive, as the body will convert GBL into GHB).

Many underground chemists spend much of their time finding precursor chemicals for these watched chemicals and producing the watched chemicals, thus avoiding detection.

As a list compiled in 1991 by Richard C. Hall III said:

Many of these chemicals are suspicious only if ordered by individuals, in large or in repeated orders. Others are suspect under any conditions.