(CNN) Life expectancy in the United States edged up for the first time in four years as the number of fatal drug overdoses and six of the 10 leading causes of death declined, according to two new government reports from the US Centers for Disease Control and Prevention's National Center for Health Statistics .

Life expectancy in the United States in 2018 was 78.7 years -- an increase of 0.1 year compared with life expectancy of 78.6 years in 2017, the NCHS reported in a study published Thursday. The 2018 estimate remains lower than the peak of 78.9 years in 2014.

At the same time, the number of drug overdoses deaths fell from 70,237 in 2017 to 67,367 in 2018 -- a drop of 4.1%, and the first such decline in decades. In 2018, the rate of overdose deaths in the United States was 4.6% lower than in 2017.

"We tried to determine why the life expectancy increased and we can actually break it down by what causes of death contributed the most to the increase," said study author Kenneth Kochanek, a researcher at the NCHS in Hyattsville, Maryland.

"That decrease in mortality from drug overdose definitely contributed to the increase in life expectancy," he said, adding, "It wasn't the leading contributor. The leading contributor was cancer. Cancer actually contributed more to that increase in life expectancy. Drug overdose would be second."

What's killing people in the United States?

The new mortality report, based on death certificate data from the National Vital Statistics System , found the 10 leading causes of death in 2017 remained the same in 2018:

Heart disease Cancer Unintentional injuries, such as drug overdose deaths and car accidents Chronic lower respiratory diseases Stroke Alzheimer's disease Diabetes Influenza and pneumonia Kidney disease Suicide

The top 10 leading causes of death accounted for 73.8% of all deaths in the United States in 2018, the most recent year for which data was available.

From 2017 to 2018, death rates decreased for six of the leading causes of death, falling by 0.8% for heart disease, 2.2% for cancer, 2.8% for unintentional injuries, 2.9% for chronic lower respiratory diseases, 1.3% for stroke and 1.6% for Alzheimer's disease, according to the new report.

The report also found that, from 2017 to 2018, death rates increased for two causes of death, rising by 4.2% for influenza and pneumonia and 1.4% for suicide. Rates for diabetes and kidney disease did not change significantly, according to the report.

The new statistics also revealed differences in death rates according to age.

Between 2017 and 2018, death rates dropped 5.1% for those between the ages of 15 and 24; 3% for ages 25 to 34; 1.4% for ages 45 to 54; 0.4% for ages 65 to 74; 1.9% for ages 75 to 84 and 0.9% for ages 85 and older, according to the new report.

The data showed that death rates for ages 35 to 44 and 55 to 64 did not change significantly.

Among infants, the report found that the infant mortality rate in the United States decreased 2.3% from about 579 infants dying per 100,000 live births in 2017 to about 566 deaths per 100,000 in 2018.

"That's another positive," Kochanek said.

"We have the increase in life expectancy. We have six of the 10 leading causes of death showing decreases, the overdose report showing a decrease in drug overdose deaths and infant mortality going down also," he said. "We're giving positive news right now."

The states with the highest drug overdose death rates

The new report on drug overdose deaths, also based on data from the National Vital Statistics System, found that the number of drug overdose deaths in the United States fell from around 22 deaths per 100,000 people in 2017 to a rate of about 21 per 100,000 in 2018.

When examining the data by state, the report found that the states with the highest rates of drug overdose deaths in 2018 were:

West Virginia, at about 52 deaths per 100,000 people Delaware, about 44 deaths per 100,000 Maryland, about 37 deaths per 100,000 Pennsylvania, about 36 deaths per 100,000 Ohio, about 36 deaths per 100,000 New Hampshire, about 36 deaths per 100,000

According to the report, the states with the lowest drug overdose death rates in 2018 were:

Texas, at about 10 deaths per 100,000 people North Dakota, about 10 deaths per 100,000 Iowa, about 10 deaths per 100,000 Nebraska, about 7 deaths per 100,000 South Dakota, about 7 deaths per 100,000

Factors driving declines in deaths

A decline in drug overdose deaths in 2018 were likely "a big part" of the uptick in life expectancy that year, "but it was also coupled with the fact that we had improvements in cancer deaths and, in particular, cardiovascular mortality, which have been falling," said Dr. Georges Benjamin, executive director of the American Public Health Association in Washington, DC. Benjamin was not involved in the new mortality reports.

He added that, in some ways, those leading causes of deaths are interlinked.

"In particular, tobacco drives cardiovascular disease and cancer, and obesity is a driving factor as well," Benjamin said, adding that if life expectancy trends continue to improve then it suggests that some of the work being done to tackle the nation's leading causes of death -- such as heart disease and cancer -- is proving successful.

"What we do see is almost across the board some slight improvement in mortality," he said. "I'm cautiously optimistic, but after having had four years of a decline in life expectancy, I worry about whether or not this is just a blip."

After decades of increases, life expectancy in the United States plateaued after 2010, then began to fall in 2014 and had been declining since , with some experts pointing to drug overdose deaths and suicides as drivers of the downturn.

Now, "after years of plateauing and declining US life expectancy, this one-year uptick is certainly welcome news. However, we need much more detailed evaluation over time to judge whether declining trends are truly being reversed in a sustained fashion," said Dr. Howard Koh, a professor of public health leadership at the Harvard T.H. Chan School of Public Health and the Harvard Kennedy School in Boston, who was not involved in the new mortality reports.

Specifically, he said that more research is needed to determine whether the improved life expectancy in the United States impacts broad populations across the country or specific demographics.

The overall decline in drug overdose deaths "is notable and must continue," Koh said.

He added that the decline reflects the impact of various factors, including the increasing availability of the medication naloxone , which can reverse an opioid overdose, and the greater attention to treatment and care as a part of public health.

"However, rising deaths from synthetic opioids -- as well as from cocaine and methamphetamine -- represent the next disturbing wave of the nation's ongoing substance use challenge," Koh said.

The overdose death rates for opioids such as heroin, oxycodone and methadone fell from 2017 to 2018 in the United States. But the rate of overdose deaths involving synthetic opioids other than methadone -- drugs such as fentanyl and tramadol -- increased 10% from 2017 to 2018, the report says. The rate of overdose deaths involving cocaine more than tripled from 2012 to 2018, and the rate of deaths involving psychostimulants such as methamphetamine increased 30% per year from 2012 to 2018, the report notes.

'Progress in health is very much possible'

The new data represent something "important" for Dr. Joshua Sharfstein, the director of the Bloomberg American Health Initiative at Johns Hopkins Bloomberg School of Public Health in Baltimore, who was not involved in the new mortality reports.

"It shows that progress in health is very much possible," he said.

Get CNN Health's weekly newsletter Sign up here to get The Results Are In with Dr. Sanjay Gupta every Tuesday from the CNN Health team.

When it comes to the drop in drug overdose deaths, "since 2014, most states have expanded their Medicaid program, allowing millions of people to access addiction treatment. In 2016, the federal government began to send additional funds to states for a wide range of services," Sharfstein said.

"These investments are starting to show some results in some areas, but death rates are still far higher than they were just a few years ago," he said. "Most people with opioid addiction still do not have access to harm reduction services or the most effective forms of treatment. Our attitude should be that we're just starting this fight, we have a lot more to do, and we won't stop until the epidemic has receded."