It’s no secret that heroin has become an epidemic in the United States. Heroin overdose deaths have risen more than sixfold in less than a decade and a half. Yet according to one of the most widely cited sources of data on drug use, the number of Americans using heroin has risen far more slowly, roughly doubling during the same time period.

Most major researchers believe that source, the National Survey on Drug Use and Health, vastly understates the increase in heroin use. But many rely on the survey anyway for a simple reason: It’s the best data they have. Several other sources that researchers once relied on are no longer being updated or have become more difficult to access. The lack of data means researchers, policymakers and public health workers are facing the worst U.S. drug epidemic in a generation without essential information about the nature of the problem or its scale.

“We’re simply flying blind when it comes to data collection, and it’s costing lives,” said John Carnevale, a drug policy expert who served at the federal Office of National Drug Control Policy under both Republican and Democratic administrations. There is anecdotal evidence of how patterns of drug use are changing, Carnevale said, and special studies conducted in various localities are identifying populations of drug users. “But the national data sets we have in place now really don’t give us the answers that we need,” he said.

Among the key questions that researchers are struggling to answer: Is the recent spike in deaths primarily the result of increased heroin use, or is it also due to the increased potency of the drug, perhaps because of the addition of fentanyl, a synthetic opioid that can kill in small doses?

“Everyone thinks they know the answer” to whether fentanyl is behind the increase in deaths among heroin users, said Daniel Ciccarone, who’s a professor at the University of California, San Francisco’s medical school and studies heroin use. “Well, show me the data. … When you don’t have data that leads to rational analysis, then what you’re left with is confusion, and confusion leads to fear, and that will lead to irrational consequences.”

Researching illicit drug use has always posed challenges. One ever-present problem: Reliable information on illegal behavior is, almost by definition, difficult to collect. But researchers point to particular limitations in data sources that would help shed light on the heroin epidemic. And they say the problems are getting worse: Some data systems that were once used by government agencies to gather information on users, consumption and illegal markets have disappeared over the past several years. Other sources that are still available are becoming more difficult to access or don’t provide a clear picture of the problem.

The National Survey on Drug Use and Health, which is an annual household survey, is sponsored by the Substance Abuse and Mental Health Services Administration, a division of the federal Department of Health and Human Services. Through roughly 70,000 interviews, the survey collects information nationwide on the use of tobacco, alcohol and illegal drugs, as well as Americans’ mental health. Experts who study illicit drugs say the survey is an important source for estimating the number of those who use alcohol, tobacco and, increasingly, cannabis (because of the normalization of marijuana use). But many consider it inadequate for calculating the number of users of harder drugs such as heroin, cocaine and methamphetamine, which carry a greater stigma. Moreover, the survey excludes people without a fixed address, meaning people who are homeless or transient — a category that includes many of the heaviest drug users. These factors lead experts to believe the survey significantly underreports the number of users of hard drugs in the U.S. (A spokesman for SAMHSA said the survey doesn’t capture certain populations, including the homeless, and acknowledged that it faces other “limitations inherent in surveys.”)

Because of the well-known shortcomings of the National Survey on Drug Use and Health and other surveys that rely on self-reporting, experts often try to combine different sources to reach a more reliable estimate of total drug use. By looking at drug production and seizures, for example, they can estimate the supply of drugs that are reaching users. Fluctuations in the street price of drugs can give hints about changes in supply and demand. Each of those estimates carries its own challenges and caveats, but in theory, putting them together should give researchers a more complete picture of drug use — and the drug market more generally — than any one data point alone.

“If you want to understand something about drug popularity, drug consumption, how much of a drug is going to be consumed in a given year, then you look at the economics,” Ciccarone said.

These more comprehensive efforts tend to yield estimates of drug use that are much higher than those based on user surveys. A 2014 analysis by researchers from the RAND Corp. that was conducted for the Office of National Drug Control Policy, for example, estimated that there were roughly 1 million daily or near-daily heroin users in the U.S. in 2010, more than 15 times the 60,000 chronic users reported in the National Survey on Drug Use and Health for the same year.

In a related report to the White House, RAND provided information on how much money heroin users were spending on the drug, estimating that expenditures were roughly $27 billion in 2010, with much of the spending driven by daily and near-daily users. That kind of analysis has become more difficult to conduct in recent years, however. RAND relied in large part on the Arrestee Drug Abuse Monitoring system, a federal program that conducted interviews with male arrestees within 48 hours of their arrest to collect information on drug use, treatment and market activity, among other topics, and then validated usage with urinalysis testing for 10 substances. The early version of ADAM, from the early 2000s, covered 35 jurisdictions. But the program was cut for a couple of years in the mid-2000s and then was revived on a smaller scale in 2007 before being eliminated in 2013. The Office of National Drug Control Policy, which took responsibility for ADAM in 2007, declined to comment on the record for this story.

Experts who used ADAM said that interviewing and testing arrestees provided insights not available through other sources. ADAM was the only national data source on individual users’ expenditures and consumption, two important metrics for understanding drug markets. The loss of the program left a particular gap in researchers’ understanding of heroin use because it had provided data on a crucial and hard-to-study group: heavy users, who often drive the overall market. Beau Kilmer, co-director of the Drug Policy Research Center at RAND, said information collected on heavy users is important for evaluating enforcement efforts and making decisions about treatment availability.

“In order to generate these estimates for heavy users, there isn’t one data source you can use,” Kilmer said. “We think about how do we combine the insights for multiple data sets, and they have their flaws, but by far the most important data set we used was ADAM.”

Another source of data that is no longer being updated is the Drug Abuse Warning Network, which was once a project of SAMHSA. It monitored drug-related hospital emergency room visits and provided insight on drug use in metropolitan areas. Rosalie Liccardo Pacula, another co-director of RAND’s Drug Policy Research Center, said a crucial advantage of DAWN was that it provided data at the local level, allowing researchers to understand drug-use patterns in different parts of the country. In an email, a SAMHSA spokesman said the agency is working with the CDC to develop a new system for collecting data from emergency rooms that will replace DAWN and combine other data sets. President Trump’s proposed 2018 budget, however, doesn’t provide funding for the new program; the spokesman said the agency will continue to “explore the viability of this approach to collect these data in the future.” In the meantime, researchers say the loss of DAWN has left a big hole in their understanding of the opioid problem.

“We know the importance of emergency rooms right now with the opioid epidemic because that’s where people are showing up,” Carnevale said. “Right now, there’s no systematic collection by the federal government for emergency room data — again, another major loss in our knowledge base.”

Researchers pointed to a third government database that could help them examine drug trends — if they had better access to the information. The Drug Enforcement Administration collects information on the drugs it obtains through seizures and undercover buys by its agents in a database known as the System to Retrieve Information from Drug Evidence. The database isn’t designed for researchers — it is an internal inventory and can therefore be influenced by which drugs law-enforcement authorities are focused on at the time — but the data has nonetheless been valuable to researchers because it contains information on the price, purity and composition of seized drugs. In some cases, Ciccarone and other researchers say, the information has become more difficult to obtain.

Pacula, who has worked with the STRIDE database and DEA data extensively, said the agency a few years ago became more restrictive with data requests after becoming concerned that researchers were improperly sharing raw data. Pacula said researchers can still access the data they need as long as they are careful about their requests; others, however, were more critical.

“The DEA has basically substantially reduced access to their data, which taxpayers are paying for that collection of data,” Carnevale said.

When asked about the access issues that the researchers described, a DEA spokeswoman wrote in an email that data entry into STRIDE had stopped in 2014 and was replaced with the National Seizure System, which isn’t available to the public and requires a formal request under the Freedom of Information Act to access. She declined to comment further on the researchers’ concerns.

Experts argue that with the heroin epidemic showing no signs of slowing down, the government should bring back data collection systems like ADAM and DAWN and ideally expand them to include more areas and broader populations. (ADAM, for example, sampled only male arrestees in urban areas.) Researchers say the cost of these programs is relatively minor, pointing to ADAM’s peak annual cost of about $10 million. That’s about a fifth of what it costs to fund the National Survey on Drug Use and Health survey each year.

The lack of reliable national data is hindering efforts to tackle the spread of heroin, experts say. Some big cities such as New York have embarked on their own data-collection efforts, something the many smaller cities and towns ravaged by heroin overdoses likely can’t afford to do. Researchers say they need the federal government to help fund and coordinate efforts to collect data from local coroner’s offices, emergency rooms and crime labs so that local officials know where and how to direct their efforts. And, more broadly, they say they need a wide range of groups — researchers, public health workers, law enforcement officers, as well as federal, state and local governments — to work together to understand the heroin epidemic and to figure out how to stop it.

“We need to have public safety, public health partnerships,” Ciccarone said. “We need the government to be forthright. We need it to think that researchers and public health officials are on the same side as the people who also want to stop the drugs.”