By Nadia Kounang CNN

(CNN) -- America's opioid epidemic continues: The latest numbers from the Centers of Disease Control and Prevention, released Friday, show that one in four drug overdoses in 2015 was related to heroin. In 1999, just 6% of all overdoses were related to the drug.

When looking at overdoses overall, opioid-related deaths represented the majority. In 2015, overdoses involving opioids represented 60% of all overdose deaths, a significant jump from about 50% in 2010. Opioids include heroin as well as drugs with a similar chemical structure, such as oxycodone and illicit synthetics like fentanyl.

Dr. Holly Hedegaard of the National Center for Health Statistics, who co-authored the study, also noted that this was the first time the number of overdose deaths in the United States exceeded 50,000. In 2010, there were 38,329 overdose-related deaths, and by 2015, that number had climbed to 52,404. By comparison, in 2015, there were 36,252 total firearm-related deaths across the country.

As with heroin, there was also a significant increase in deaths involving synthetic opioids such as fentanyl, the drug that was blamed for pop star Prince's death. In 2010, these types of drugs were involved in just 8% of all overdose deaths, and by 2015, they were involved in 18% of all overdose deaths.

While there were increases in heroin and synthetic drug-related deaths, there was a drop in overdose deaths involving natural and semisynthetic opioid analgesics, including prescription drugs like oxycodone and hydrocodone. Although these drugs were involved in 29% of drug overdose deaths in 2010, they represented 24% of all drug overdose deaths in 2015.

This shift in numbers may in part be due to a change in user habits, with some starting out with prescription drugs and moving on to heroin because of cost and crackdowns on illegal use of prescription drugs. However, Dr. Andrew Kolodny, co-director of Brandeis University's Opioid Policy Research Center, said that switching is only part of the story.

"Starting in 2011, overdoses involving heroin has really skyrocketed. There's a really good chance the increase involving heroin has to be involved with fentanyl," he said.

Search for solutions

In an attempt to stem the tide of opioid-involved deaths, state and federal governments have implemented new laws and regulations directed at the epidemic. This month, New Jersey Gov. Chris Christie signed a law that would limit initial prescriptions of opioids to just five days. In Arizona, a similar law limits prescriptions to seven days.

The Drug Enforcement Administration has listed a number of fentanyl variations or analogues as schedule I, drugs that have "no currently accepted medical use and a high potential for abuse."

Dr. Larissa Mooney, director of the University of California Los Angeles Addiction Medicine Clinic, said the new study highlighted the need for opioid addiction treatment. "We need to improve access to treatment and remove barriers," she said.

When Congress passed the 21st Century Cures Act last year, it also dedicated $1 billion toward fighting the epidemic, including expanding buprenorphine treatment, a medication-assisted treatment for opioid dependency.

Unlike methadone, which can be administered only in specific settings, buprenorphine -- commonly used under the brand name Suboxone -- can be provided outside a clinical setting, which can make it more easily accessible.

But while medical-assisted treatment is considered the gold standard, as with any treatment, it isn't 100% effective.

A study published this week in the journal Addiction found that 43% of all buprenorphine users filled an opioid prescription during treatment and that another 67% filled an opioid prescription after treatment. However, buprenorphine can also be prescribed to deal with chronic pain, and the study wasn't able to determine which users were trying to treat their dependency versus to treat pain.

Medically assisted treatment

"The bottom line: It's no surprise that some people receiving buprenorphine are also receiving prescriptions of other opioids, but we were surprised by the number of patients receiving buprenorphine and other opioids," said study author Dr. Caleb Alexander. Alexander is Associate Professor of Epidemiology and Medicine at the Johns Hopkins Bloomberg School of Public Health.

Alexander pointed out that the study did not aim to assess the effectiveness of buprenorphine but rather "raises (the) question about how we can improve the quality and continuity of this treatment. "

The study looked at 38,096 buprenorphine users between January 2010 and July 2012, with an average treatment length of 55 days.

"When prescribed appropriately," Kolodny said, "more than 75% of patients do very well" on buprenorphine. He noted that success from buprenorphine treatment was based on long-term use of at least a year.

In addition, since 2012, awareness of the opioid epidemic has been visibly increased. Last year, the CDC issued new guidelines to physicians on prescribing opioids, including recommending against using narcotics as a first-line therapy for chronic pain.

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