he started his career as a family physician in turnhout, but in addition he was interested in business too. in 1921, he met ladislas richter in vienna, who was the son of gedeon richter, who owned a pharmaceutical factory in budapest, hungary. in 1933, he acquired the right to distribute the pharmaceutical products of richter, the hungarian pharmaceutical company, for belgium, the netherlands and belgian congo. on 23 october 1934, he founded the "n.v. produkten richter" in turnhout. in 1937 constant janssen acquired an old factory building in the "statiestraat 78" in turnhout for his growing company, which he expanded during world war ii into a four-storey building. in 1938 he closed his practice as a family physician and concentrated his effort of the expansion of his pharmaceutical business. his wife managed the production, quality control, and the administration of the company.