What is the term for a substance that slows down a chemical reaction?

A substance that slows or prevents a chemical reaction is called an inhibitor or a negative catalyst. It is the opposite of a catalyst, which is a substance that increases the rate of a reaction.

An example of an inhibitor is acetanilide. When it is added to hydrogen peroxide, it slows down the decomposition of the solution. Inhibitors are also used alongside catalysts, such as enzymes, to reduce their effectiveness. Poisons and some drugs can act as enzyme inhibitors. Enzyme inhibitors can be classified as specific or nonspecific. A specific inhibitor works on a single type of enzyme, and a nonspecific inhibitor has the same effect on all enzymes.