Hans worked as a mission doctor in India and Ghana for five years and joined the World Health Organization (WHO) in 1985. From 1999 to 2008, he was the Secretary of the WHO Model List of Essential Medicines and from 2004 to 2011, he was WHO Director for Essential Medicines and Pharmaceutical Policies in Geneva.
Hans has advised over 60 low- and middle-income countries (LMICs), including China, Ghana, India, Iran, Maldives, Morocco, Nepal, South Africa, Sri Lanka and Uganda, on the development of their national medicines policies. Hans is the editor of several WHO books and has written over 80 scientific papers on essential medicines policies. His current research interests include promoting access to insulin in LMICs; monitoring access to expensive essential medicines as part of the right to health; and monitoring the implementation of national medicine policies at country level.