COPENHAGEN—Novo Nordisk shares fell as much as 16 percent on Monday after the Danish drugmaker was hit by U.S. supply issues for its new obesity drug as it seeks to establish a market foothold before the launch of a rival drug by Eli Lilly. The obesity market has proved difficult for pharmaceuticals companies, but Novo made a breakthrough with its Wegovy drug. The company hopes the drug, which helps to achieve weight loss of 17 percent on average over almost two years, will offset growing pressure on its core insulin business. In a stock announcement after Friday’s market close, Novo said that a contract manufacturer filling syringes for pens to inject the drug had halted deliveries and manufacturing temporarily after issues relating to good manufacturing practice. The Danish drugmaker was already facing separate supply constraints after being “overwhelmed” by initial uptake of the drug. As a consequence, fewer new patients …