More than half of federal prisoners are drug offenders. They are the people most likely to benefit from the Senate's historic criminal justice bill, introduced Thursday. There's a good reason for that: The war on drugs has been an enormous, costly failure.

The point of the war on drugs, particularly international interdiction and domestic law enforcement efforts, is to go after the supply of drugs. The idea: If you hit the drug trade, supply will shrink, prices will go up, and drug habits will become unaffordable.

Yet since the US stepped up its drug war in the 1980s, federal data shows the opposite occurring, with prices for the major illicit drugs — excluding marijuana, which has never been very expensive — falling. Heroin is perhaps the prime example, with its price completely collapsing over the past several decades.

The numbers we have on supply back this up. A 2015 report by the UN Office on Drugs and Crime (UNODC) found that the production of opium, which is used to make heroin, has generally trended up since the late 1990s — with a brief drop in 2001 after the invasion of Afghanistan, the world's largest opium producer.

Not only has the drug war failed to reduce the global production of opium, but, the UNODC report notes, production in 2014 reached its second-highest levels since the 1930s. That's despite concerted US efforts in Afghanistan to crack down on the drug: The US spent $7.6 billion between 2002 and 2014 to eliminate Afghani poppy cultivation.

"The bottom line — record opium cultivation and production — clearly shows we are not winning the war on drugs in Afghanistan," John Sopko, the US military's watchdog in Afghanistan, said in a May 5 speech. "Of course, the US government announced its own war on drugs in 1971, almost 45 years ago, and we haven't won that, either."

So despite billions in spending, the war on drugs never appeared to have a significant impact on the supply or price of heroin — one of the drugs currently tearing through large parts of the US as part of a prescription painkiller and heroin epidemic.

That doesn't necessarily mean the war on drugs had zero impact. It's possible that it reduced supply and raised prices, just not enough to reverse larger trends.

But that's exactly the problem: If the costly drug war wasn't enough to reverse these trends, then maybe it's worth trying something else. So the Senate wants to reduce penalties for drugs, while the White House is looking toward a public health approach to drugs.