Update: This explainer is no longer being updated. For the new, up-to-date explainer on the opioid epidemic, click here.

In 2015, more Americans died of drug overdoses than any other year on record: more than 52,000 deaths in just one year. That's far more than the more than 38,000 who died in car crashes, the more than 36,000 who died due to gun violence, and the more than 43,000 who died due to HIV/AIDS during that epidemic's peak in 1995.

But this latest drug epidemic is not driven primarily by illicit drugs. It began with a legal drug: opioid painkillers.

Back in the 1990s, doctors agreed — and many still do — that America has a serious pain problem: Tens of millions of Americans experienced debilitating pain, and it was left untreated. So they looked for a solution — and, fueled by a misleading marketing push from pharmaceutical companies, landed on opioid painkillers, widely known by brand names such as OxyContin, Percocet, and Vicodin. The drugs proliferated.

But this led to unintended, devastating results. Prescription painkiller misuse went up, and overdose deaths linked to the drugs did as well. Then, as policymakers and doctors took notice of widespread painkiller misuse, they pulled back access to the drugs. But federal data shows many of these drug users didn't just quit the drugs altogether — some instead moved to a lower-cost, more potent opioid, heroin, and some are reportedly moving to the even stronger synthetic opioid, fentanyl.

As a result, more than 33,000 deadly drug overdoses in 2015 — almost two-thirds of drug overdoses that year — involved some type of opioid.

It's a big public health crisis. Surprisingly, some policymakers are treating it as a public health crisis: Whereas policy responses to drug epidemics tend to focus on harsh tactics typical of the war on drugs (like increased prison sentences for drug possession), the current crisis is also fueling calls for more access to treatment and other public health programs.

The response shows how America's drug policies are changing. But to understand how the changes relate to the opioid epidemic, it's important to start from the beginning, with how the current drug crisis began.

A misleading marketing campaign from drug companies led to the epidemic

America has a pain problem. About 100 million US adults suffer from chronic pain, according to a 2011 report from the Institute of Medicine. This might seem like an excessive number — roughly one-third of all Americans — but it includes everyone in the chronic pain spectrum, from the silent sufferer who deals with constant back pain to the patient who can no longer move because the pain all over her body is just too much.

There isn't a single medication that will relieve all pain for all patients. But there was a huge push in the 1990s and 2000s — from drug companies in particular, the federal government's flawed "Pain as the Fifth Vital Sign" campaign, and pharmaceutical-backed advocacy efforts — that doctors do something about pain.

As Keith Humphreys, drug policy expert at Stanford University, explained, there's simply no good scientific evidence that opioid painkillers can actually treat long-term chronic pain as patients grow tolerant of opioids' effects, but there's plenty of evidence that prolonged use can result in very bad complications, including a higher risk of addiction, overdose, and death.

But pharmaceutical companies saw an opportunity for profit, and they marketed opioids to doctors as a safer, more effective way to treat pain than other medications on the market.

The result: Drug companies made a lot of money as people got addicted and died.

Pharmaceutical companies' claims of safety and efficacy were, of course, inaccurate. And Purdue Pharma, producer of the opioid OxyContin, later paid hundreds of millions of dollars in fines for its false claims. Opioid painkillers carry a significant risk of addiction and overdose, especially for long-term users who build up a tolerance of the high and use more and more of the drug without building as much resistance for the respiratory effects that lead to overdose.

But many doctors, under pressure to treat pain more seriously, bought into the messaging from those decades and prescribed a ridiculous amount of painkillers to patients. In 2012, US physicians wrote 259 million prescriptions for opioid painkillers — enough to give a bottle of pills to every adult in the country. The drugs proliferated, landing in the hands of not just patients but also teens rummaging through their parents' medicine cabinets, other family members and friends of patients, and the black market.

As state and federal governments became aware of the problem, they began going after doctors and pharmacists who provided painkillers too leniently, threatening them with incarceration and the loss of their medical licenses. In 2014, the Drug Enforcement Administration reclassified some opioid painkillers from Schedule 3 to the more restrictive Schedule 2, limiting access for both patients and doctors.

Ideally, doctors should still be able to get painkillers to patients who truly need them (and they can work for some individual chronic pain patients) — after, for example, evaluating the patient's history of drug addiction. But doctors who weren't conducting even such basic checks are now being told — not just through the crackdown, but by health care organizations and public education campaigns — to give more thought to their prescriptions.

Doctors don't always have to resort to opioids to treat pain as a serious medical issue. There are alternatives, such as special exercises, physical therapy, surgeries, and lifestyle changes. There's also some evidence for medical marijuana, which studies have shown to be effective at treating chronic pain and averting opioid deaths. And in some cases, some people suffering from pain may just have to find ways to live with it, because the risks of opioids simply outweigh the benefits — especially since, again, there's no good evidence that opioids can effectively treat chronic pain.

"If you tried all of that and none of it worked, what we're left with is learning to live with pain," Anna Lembke, a Stanford psychiatrist and author of Drug Dealer, MD, told me. "How can we create a lifestyle that you're comfortable with while knowing that you’re not going to be able to get rid of the pain? That becomes a spiritual, existential question for a lot of people — a very profound one that takes a lot of thoughts and efforts to have questions about. It's not something you can do in five minutes."

Despite increased awareness and the crackdown, there are still signs of some doctors doing a lot of overprescribing. A 2015 Centers for Disease Control and Prevention report found that a small minority of prescribers are responsible for most opioid prescriptions, although there's a lot of variation from state to state. For example, the top 1 percent of prescribers wrote one in four opioid prescriptions in Delaware in 2013, while the top 1 percent of prescribers wrote one in eight such prescriptions in Maine.

Still, the crackdown has slowed the rise in painkiller overdose deaths. But it also likely led to an increase in other kinds of opioid deaths.

Painkiller deaths have plateaued, but heroin and fentanyl deaths have skyrocketed

When opioid users couldn't fulfill their cravings with painkillers, many turned to an opioid that is, despite its status as an illegal substance, cheaper and more accessible than the legal medicine: heroin. And increasingly, there's evidence that people are turning to yet another drug: fentanyl, a synthetic opioid that's often manufactured illegally for non-medical uses.

The data tells the story: As overdoses from traditional painkillers plateaued in the past few years, overdose deaths linked to heroin overtook overdose deaths linked to commonly prescribed painkillers, and overdose deaths linked to synthetic opioids like fentanyl rapidly rose.

Though all heroin users didn't necessarily start with painkillers, it's the transition from painkillers to heroin and fentanyl, experts say, that led to much of the dramatic spike in heroin and fentanyl deaths.

Studies back this up: A 2014 study in JAMA Psychiatry found many painkiller users were moving on to heroin, and a 2015 CDC analysis found people who are addicted to prescription painkillers are 40 times more likely to be addicted to heroin.

Heroin is even deadlier than opioid painkillers; it's far more potent, and more addictive. So even if a small number of painkiller users moved on to heroin, it would still, on a per-person basis, lead to far more deaths.

That's even more true for fentanyl, which is even more potent — and deadlier — than heroin and becoming more widespread as drug users seek out alternatives to painkillers.

What's worse, opioid users tend to mix the drugs with other substances — like alcohol and cocaine — that exacerbate the risk of an overdose. A 2003 study found roughly half of heroin-related deaths involved alcohol, and the CDC found that 31 percent of prescription painkiller–linked overdose deaths in 2011 were also linked to benzodiazepines, a legal anti-anxiety drug.

So as people used painkillers and moved on to heroin and fentanyl, they continued using all these other substances that made their risk of overdose much, much higher — and it's showing in the numbers as heroin and fentanyl overdose deaths spike.

That doesn't mean cracking down on painkillers was a mistake. It appeared to slow the rising number of painkiller deaths, and it may have prevented doctors from prescribing the drugs — or letting them proliferate — to new generations of people who'd develop drug use disorders. So the crackdown did lead to more heroin deaths, but it will hopefully prevent future populations of drug users, who could have suffered even more overdose deaths.

That's why, though they knew it could lead to a temporary spike in heroin use, state and federal agencies came down on painkillers.

The rise in heroin deaths wasn't unexpected

The results of a government crackdown on opioid painkiller prescriptions were long a concern in medical circles and among drug policy experts, who warned it could lead to a rise in heroin use.

"We always were concerned about heroin," Kevin Sabet, a former senior drug policy official for the Obama administration, told the Huffington Post in 2015. "We were always cognizant of the push-down, pop-up problem. But we weren't about to let these pill mills flourish in the name of worrying about something that hadn't happened yet. … When crooks are putting on white coats and handing out pills like candy, how could we expect a responsible administration not to act?"

The unintended consequence is a very typical result of governments' anti-drug efforts. It's called the balloon effect: When the government cracks down on one source of supply for drugs, people don't just stop using. Instead, they find another source — and the cycle continues.

The balloon effect has been observed not just with the crackdown on opioid painkillers, but with anti-drug efforts in Latin American countries. After the governments there cracked down on the illicit drug trade in the 1990s and 2000s, it simply shifted to other parts of Central and South America. This effect is one of the primary reasons the war on drugs has failed to significantly curtail drug trafficking.

As Sabet acknowledged, the government knew this was a possibility with opioids. But the feds still thought it was worth cutting off the supply of painkillers to prevent doctors and pharmacists from creating even more generations of problematic painkiller users.

But this didn't deal with all existing opioid users, who are now dying by the tens of thousands each year. To deal with that, policymakers are resorting to a mix of policies — some "tough on crime," others focused on public health programs.

The opioid epidemic has led to more emphasis on public health programs

Federal and state governments have, particularly since the 1970s, tended to respond to drug epidemics with "tough on crime" measures.

President Richard Nixon launched the modern war on drugs in 1971 in part as a response to the heroin epidemic of the time, which Nixon characterized as a "deadly poison in the American life stream." And President Ronald Reagan massively increased drug penalties in the 1980s as part of a response to the crack cocaine epidemic.

Drug policy experts widely agree that the "tough on crime" policies were too harsh and largely ineffective. Although these policies were meant to inflate the price of heroin, the drug's cost actually plummeted by more than 90 percent from 1981 to 2007. Illicit drug use, meanwhile, rose through the 2000s. And the mere existence of the current heroin crisis shows how ineffective these policies are — since they couldn't prevent a full-blown epidemic.

Nonetheless, some lawmakers are doubling down on "tough on crime" policies in an attempt to deal with the opioid epidemic. Some states, for example, are charging drug dealers and suppliers with murder if the drug leads to a deadly overdose. In conservative states, like Louisiana and Indiana, officials have actually increased punishments for heroin dealing.

But in many other states, the opioid epidemic is inviting a very different approach: one focused on public health. Dozens of states have pulled back their harsh drug laws over the past several years. Michael Botticelli, the nation's drug czar under the Obama administration and essentially the leader of the war on drugs at the time, said in 2015 that "we can't arrest and incarcerate addiction out of people."

Race and class may play a role in the softer approach. Unlike the heroin epidemic of the 1960s and 1970s and the crack cocaine epidemic of the 1980s, the current epidemic isn't a problem left to mostly urban, low-income, and black areas; the places reporting the biggest struggles with opioids — particularly in West Virginia and New Hampshire — tend to be rural and suburban, white, and middle class.

Budget concerns likely play a role, too. Faced with growing prison costs and the failure of the drug war to significantly curtail drug addiction, states have cut back on old "tough on drugs" tactics by, for example, pushing low-level offenders to specialized drug courts that attempt to put drug users in treatment and rehabilitation instead of jail and prison. (Although this approach has been heavily criticized.)

These are policy changes that should, at least in theory, benefit anyone with a drug problem who would have been doomed to prison in the past. But people with opioid use disorders will be among the first to claim the results of reform due to the ongoing epidemic.

Whatever the cause, the public health approach is in line with both public and expert opinion. Polls show that most Americans prefer treating drugs as a public health issue, not a criminal one. And many experts, including the International Narcotics Control Board, have asked for a greater focus on public health policies to curtail demand for drugs.

The public health responses emphasize treatment and prevention

In the places that have responded to the opioid epidemic through a public health approach, policymakers have focused on boosting access to harm reduction policies and drug treatment.

Some jurisdictions have embraced harm reduction strategies: They acknowledge that some people will always use drugs, but there are steps policymakers can take to stop that drug use from turning deadly. Various state legislatures controlled by Democrats and Republicans have, for example, passed laws allowing police and even private individuals to carry naloxone, which reverses opioid overdoses. Others have tried clean-needle exchanges, which let drug users obtain clean needles if they trade in dirty ones — to avoid the risk of HIV or hepatitis infection.

Meanwhile, some lawmakers are giving more attention to drug treatment.

The federal government, for one, has stepped up general spending on treatment and prevention programs over the past few years. The Obama administration dedicated $2.5 million in 2015 to fight heroin use. The Department of Health and Human Services also unlocked more than $100 million for drug treatment in 2016. And Congress in late 2016 approved $1 billion over two years to combat the opioid epidemic.

"The main distinction with this plan is the general acknowledgment that substance use is a public health issue," White House drug czar Michael Botticelli told me in 2014, speaking to his office's budget. "We can't arrest our way out of the problem, and we really need to focus our attention on proven public health strategies to make a significant difference as it relates to drug use and consequences to that in the United States."

This is all trying to address a serious gap in health care: According to 2014 federal data, at least 89 percent of people who meet the definition for a drug use disorder don't get treatment. (If anything, that's likely an underestimate: Federal household surveys leave out incarcerated and homeless individuals, who are more likely to have serious, untreated drug problems.)

There are many reasons for this gap, including stigma against drug users and addiction treatment. But one key factor is that there simply aren't enough treatment options and programs out there — so often people with drug use disorders have to wait weeks or months just to get into care.

One big concern: boosting access to medication-assisted treatments for addiction like methadone and Suboxone, opioids that when taken as prescribed can supplant someone's painkiller or heroin use without a similar risk of misuse or overdose. Decades of research have deemed these drugs effective. The Centers for Disease Control and Prevention, National Institute on Drug Abuse, and World Health Organization acknowledge their medical value.

So different levels of government have put more resources toward treatment and prevention programs.

That doesn't mean that the old "tough on crime" approach is totally gone. The federal government and many states still impose long prison sentences for heroin-related crimes, including possession.

But it is remarkable that the current epidemic hasn't led to a response focused solely on the criminal justice system, as previous drug crises have. For drug policy reformers, it's a small step forward — a sign that drug policies are potentially shifting to a less punitive approach overall.