His most famous work was a pamphlet on birth control, The Fruits of Philosophy, or the Private Companion of Young Married People (published anonymously in 1832): Knowlton was prosecuted a number of times for the publication, resulting in his being sentenced to three months hard labour in December 1832. The pamphlet reached its widest circulation in the late 1870s, as the result of the widely publicised trial of Charles Bradlaugh and Annie Besant for republishing the pamphlet in Britain. Knowlton is considered a pioneer of the Freethinkers movement.