The list of essential medicines is a listing of drugs compiled by the WHO. The list contains a number of drugs, which cover the basic needs of a society. The drugs on the list should be easy to get, and affordable. The list was first published in 1977. It is reviewed every two years. In 2003, 156 countries have joined the effort. As of 2016, over 300 different drugs and vaccines are on the list. About 95 percent of the drugs and substances on the list are not covered by patents, which means that generic drugs using the substance indicated can be produced easily.