Drug patents create short-term monopolies. The deal is simple: the drug inventor makes its formula public and in exchange is ＿＿（36）＿＿ a competition-free run at the market, lasting up to 20 years. This gives pioneers time to 37 the costs of researching and developing new compounds, vital when creating a new medicine can cost up to $5 billion. The patent guarantees a 38 return, meaning companies have both the means and the incentive to keep innovating. When the patent reaches its expiry date, the comfortable monopoly evaporates, replaced by 39 competition. 40 have three ways of defending themselves. They are marketing, nudging customers towards newer drugs still protected by patent, and paying the competitors not to produce.