Séguin's and Courtois' opium research came to an end at the École Polytechnique in 1804. Courtois then went to his father's business in Paris for making potassium nitrate. By 1805 his father's business was failing and he was put in debtors' prison until the end of 1807. Courtois managed the family business meanwhile until his father was released. No details are known of the demise of Courtois' father after his release. Courtois is recorded as a Parisian businessman in 1806 as a "salpêtrier." In 1808 Courtois married the daughter of a Parisian hairdresser. Records show he continued to operate the family saltpeter factory until 1821, with the possible exception of years 1815, 1816, and 1817.