Zainabu Sesay, at her home in Sierra Leone, receives hospice care, but no morphine is available to ease the pain of breast cancer.

The World Health Organization estimates that 4.8 million people a year with moderate to severe cancer pain receive no appropriate treatment. Nor do another 1.4 million with late-stage AIDS. For other causes of lingering pain — burns, car accidents, gunshots, diabetic nerve damage, sickle-cell disease and so on — it issues no estimates but believes that millions go untreated.

Figures gathered by the International Narcotics Control Board, a United Nations agency, make it clear: citizens of rich nations suffer less. Six countries — the United States, Canada, France, Germany, Britain and Australia — consume 79 percent of the world’s morphine, according to a 2005 estimate. The poor and middle-income countries where 80 percent of the world’s people live consumed only about 6 percent.