A class of synthetic drugs has replaced heroin in many major American drug markets, ushering in a more deadly phase of the opioid epidemic.

New numbers Thursday from the Centers for Disease Control and Prevention show that drug overdoses killed more than 70,000 Americans in 2017, a record. Overdose deaths are higher than deaths from H.I.V., car crashes or gun violence at their peaks. The data also show that the increased deaths correspond strongly with the use of synthetic opioids known as fentanyls.

Since 2013, the number of overdose deaths associated with fentanyls and similar drugs has grown to more than 28,000, from 3,000. Deaths involving fentanyls increased more than 45 percent in 2017 alone.

Drug overdose deaths, 1980 to 2017

“If we're talking about counting the bodies, where they lie, and the cause of death, we're talking about a fentanyls crisis,” said Jon Zibbell, a senior public health scientist at the research group RTI International.

The recent increases in drug overdose deaths have been so steep that they have contributed to reductions in the country’s life expectancy over the last three years, a pattern unprecedented since World War II. Life expectancy at birth has fallen by nearly four months, and drug overdoses are the leading cause of death for adults under 55.

“The idea that a developed wealthy nation like ours has declining life expectancy just doesn’t seem right,” said Robert Anderson, the chief of mortality statistics at the C.D.C., who helped prepare the reports. “If you look at the other wealthy countries of the world, they're not seeing the same thing.”

In a separate report, the C.D.C. also documented a 3.7 percent increase in the suicide rate, another continuation of a recent trend. The increases were particularly concentrated in rural America, and among middle-aged women, though the suicide rate for men remains higher than that for women at every age.

Recent federal public policy responses to the opioid epidemic have focused on opioid prescriptions. But several public health researchers say that the rise of fentanyls requires different tools. Opioid prescriptions have been falling, even as the death rates from overdoses are rising.

“Fentanyl deaths are up, a 45 percent increase; that is not a success,” said Dr. Dan Ciccarone, a professor of family and community medicine at the University of California, San Francisco. “We have a heroin and synthetic opioid epidemic that is out of control and needs to be addressed.”

Synthetic drugs tend to be more deadly than prescription pills and heroin for two main reasons. They are usually more potent, meaning small errors in measurement can lead to an overdose. The blends of synthetic drugs also tend to change frequently, making it easy for drug users to underestimate the strength of the drug they are injecting. In some parts of the country, drugs sold as heroin are exclusively fentanyls now.

Fentanyl’s contribution to the overdose death rate in selected states, 2015 to 2017

The trends in overdose deaths vary widely across the country. The epidemic has been strongest in Northeast, Midwest and mid-Atlantic states. In the West, where heroin is much less likely to be mixed with fentanyls, overdose rates are far lower. Data from the C.D.C. indicate that a state’s overdose trend closely tracks the number of fentanyl-related deaths.

Despite the sharp recent increases in drug-related deaths, some early signs suggest that 2017 could be the peak of the overdose epidemic. Preliminary C.D.C. data show death rates leveling off nationally in the early months of this year, though there is still a lot of local variation. Several states and cities have embarked on ambitious public health programs to reduce the deadliness of drug use and connect more drug users with treatment, and some of those changes may be bearing fruit, for instance in Dayton, Ohio, where local officials have worked hard to push down the overdose rate. And in a ruling with implications for prisons and jails nationwide, a federal judge in Massachusetts this week ordered a jail to offer an addicted man access to methadone.

“What’s encouraging to me is that it’s sort of an all-hands-on-deck problem, and we’ve got all hands on deck,” said Anna Lembke, an associate professor of psychiatry at Stanford, and the author of a book on how medical practice contributed to the opioid epidemic.