Hans worked for five years as a mission doctor in India and Ghana 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 sixty low- and middle-income countries, including South Africa, India and China, and more recently Morocco, Ghana, Uganda, Iran, Sri Lanka and Nepal, on the development of their national medicines policies. Hans is the editor of several WHO books and wrote over 80 scientific papers on essential medicines policies. His current research interests include promoting access to insulin in low- and middle-income countries, monitoring access to expensive essential medicines as part of the right to health, and monitoring the implementation of national medicine policies at country level.