reactant: A substance participating in a chemical reaction, especially a directly reacting substance present at the initiation of the reaction.
reagent: A substance used in a chemical reaction to detect, measure, examine, or produce other substances.
For example, hydrochloric acid is the chemical reagent, that would cause the reactant calcium carbonate to release carbon dioxide. Naming as a reactant or reagent is a matter of convention or perspective.
I am a spanish/english translator and I need to know if the correct collocation to mean the test conducted to recognize cocaine during customs controls requires a "reactant of cocaine" or a "reagent of cocaine" or something different. My interest is only terminological, my mother tongue is Spanish and I am translating into English, so you, English native speakers and physicists, will certainly be able to help me. Thank you.
'Reagent' is more appropriate (or more common) but 'reactant' would not be incorrect in this case. Most test kits contain 'reagents' as you could discern from many chemical suppliers' descriptions of their test kits.