The use of hard drugs, meanwhile, has remained roughly stable over the last two decades, rising by a few percentage points in the 1990s and declining by a few percentage points over the last decade, with consumption patterns moving from one drug to another according to fashion and ease of purchase.

For instance, 2.9 percent of high school seniors admit to having tried cocaine in the last year, just slightly less than in 1992. About 15 percent of seniors said they abused a prescription drug last year. Twenty years ago, prescription drug abuse was not even consistently measured.

The only dimension along which the war on drugs might be conceived as a success is political. If you ask Americans how concerned they are about drugs, they will give you roughly the same answer they have given for years: not so much.

In a Gallup poll, only 31 percent of Americans said they thought the government was making much progress dealing with illegal drugs, the lowest share since 1997. But fewer people say they worry about drug abuse than 10 years ago. Only 29 percent of Americans think it is an extremely or very serious problem where they live, the lowest share in the last decade.

But the government has spent $20 billion to $25 billion a year on counternarcotics efforts over the last decade. That is a pretty high price tag for political cover, to stop drugs from becoming a prominent issue on voters’ radar screen. It becomes unacceptably high if you add in the real costs of the drug wars. That includes more than 55,000 Mexicans and tens of thousands of Central Americans killed by drug-fueled violence since Mexico’s departing president, Felipe Calderón, declared war six years ago against the traffickers ferrying drugs across the border.

And the domestic costs are enormous, too. Almost one in five inmates in state prisons and half of those in federal prisons are serving time for drug offenses. In 2010, 1.64 million people were arrested for drug violations. Four out of five arrests were for possession. Nearly half were for possession of often-tiny amounts of marijuana.

Harry Levine, a sociologist at Queens College of the City University of New York, told me that processing each of the roughly 85,000 arrests for drug misdemeanors in New York City last year cost the city $1,500 to $2,000. And that is just the cost to the budget. Hundreds of thousands of Americans, mostly black and poor, are unable to get a job, a credit card or even an apartment to rent because of the lasting stigma of a criminal record for carrying an ounce of marijuana.