 He attained a PhD in chemistry at the University of Zurich in 1925, and was an assistant of Paul Karrer, who won the Nobel Prize in Chemistry in 1937.After a stay in the USA, Joos returned to Switzerland in 1932 and started his own laboratory where he discovered the analgesic properties of phenazopyridine (later branded as Pyridazil).In 1936 he created the pharmaceutical company Cilag (Chemical Industrial Laboratory AG).