Deaths linked to opioids are skyrocketing, and so is the number of Americans abusing the drugs, experts say.
Two million Americans were dependent on prescription opioids in 2014, when the most recent data was available, federal statistics show.
Overdoses linked to opioids killed more than 28,000 people nationwide two years ago, according to the Centers for Disease Control and Prevention. The deaths include those involving heroin and prescription drugs such as oxycodone and hydrocodone.
Here are four things to know about opioids:
What are opioids?
Opioids are drugs that reduce pain by switching off pain receptors in the brain.
They are among the most widely prescribed drugs in the United States, the CDC says, fueling an epidemic that starts at the doctor's office and spills onto the streets.
Doctors prescribe them so much, federal officials published national guidelines last month urging medical practitioners to be more judicious when recommending them to patients.
How bad are opioid overdoses in the U.S.?
The statistics are grim.
Most drug overdose deaths -- at least six out of 10 -- involve an opioid, federal statistics show.
Drug overdoses are the leading cause of unintentional death for Americans, and most started off with a prescription for pain, according to the CDC.
"Overdose deaths involving prescription opioids have quadrupled since 1999, and so have sales of these prescription drugs," the CDC says.
While the amount of prescription opioids has grown fourfold, the amount of pain reported by Americans has not changed much, it says.
What drugs are considered opioids?
Prescription opioids include painkillers such as morphine, oxycodone and hydrocodone, according to the CDC.
Others, such as the synthetic pain reliever fentanyl, are used for patients suffering from advanced terminal illness such as cancer.
Heroin is an illegal opioid derived from the same poppy plant as Food and Drug Administration-approved opioid pills. Use of heroin has skyrocketed nationwide due to a lower price and wide availability on the street.
Why are opioids dangerous?
While heroin is an illegal opioid and other opioid pills are prescribed, they both bind to the same group of receptors in the brain.
They all produce an increased pain tolerance and a sense of euphoria. All opioids trigger a craving when the drug is absent. Individuals who use or take opioids build a tolerance, so they need higher doses for the same effect over time.
Many heroin users first became addicted to prescription pills. It's a cheaper alternative, about one-tenth the price, when they can no longer get pills.
There's a ripple effect around heroin use: In addition to the risk of death and overdoses, heroin addiction comes with a whole new set of problems, such as infections of the heart lining and valves, and rheumatological diseases.
Heroin users are also more likely to suffer from HIV, hepatitis and other blood diseases because users might inject the drug with a shared needle.
The availability of the drugs on the street has not helped much, fueling an epidemic of overdoses.
CNN's Nadia Kounang and Dr. Sanjay Gupta contributed to this report