dr. jan constant janssen (18 september 1895 in belgium, vlimmeren – 15 april 1970 in antwerp) was a belgian physician and businessman. he was the third child (out of four) of adriaan victor janssen (1854–1942) and anna catharina eelen (1855–1929). he went to high school in hoogstraten and studied medicine at the catholic university of leuven and the university of ghent. on 16 april 1925, he married margriet fleerackers (turnhout, 5 january 1897-vosselaar, 23 september 1973), together they had four children. their oldest son, paul janssen, would become one of the most successful scientists in pharmaceutics.