This article explores the role of testing in the allocation of royal monopoly privileges for drugs in eighteenth-century France by following the multi-generational fortunes of a single "secret remedy" from 1713 to 1776: the poudre fébrifuge of the Chevalier de Guiller. On at least five occasions, this drug was tested on patients in order to decide whether it should be protected by a privilege and whether or not its vendors should be awarded lucrative contracts to supply it in bulk to the French military. Although efforts were made early in the century to test the drug through large-scale hospital trials and to relegate privilege granting to a bureaucratic commission, the case of the poudre fébrifuge instead suggests that military expediency and relatively small-scale trials administered personally by royal practitioners remained decisive in determining whether or not a drug received a monopoly privilege or a military contract.