(CNN) -- Using heroin can kill you, but it may not be in the way you think.
Drug overdose deaths in the United States have risen steadily since 1970. Painkillers actually kill more Americans than heroin and cocaine combined, according to the Centers for Disease Control, but heroin is still one of the No. 1 killers of illegal drug users. Only one in 10 heroin overdoses ends in death.
Overdose deaths from heroin have increased recently, and heroin use is also on the rise. In 2011, 4.2 million Americans over the age of 11 had tried heroin at least once, according to the National Institute on Drug Abuse.
An estimated 23% of them will become addicts. And it's addicts who die more frequently than new users, studies show.
How heroin works
Heroin is most often mixed with water and injected. Injecting it minimizes the lag time between when the drug is taken and effects are felt -- with injection, the effects are almost immediate.
It can also be smoked, snorted or eaten, but smoking or eating destroys some of the drug and mutes its effects.
When someone takes heroin there is an immediate rush. Then the body feels an extreme form of relaxation and a decreased sense of pain.
What's happening inside the body is the heroin is turning into morphine. Morphine has a chemical structure similar to endorphins -- the chemicals your brain makes when you feel stressed out or are in pain. Endorphins inhibit your neurons from firing, so they halt pain and create a good feeling.
Morphine, acting like your endorphins, binds to molecules in your brain called opioid receptors. When those receptors are blocked, that creates a high.
Why you die
Most people die from heroin overdoses when their bodies forget to breathe.
"Heroin makes someone calm and a little bit sleepy, but if you take too much then you can fall asleep, and when you are asleep your respiratory drive shuts down," said Dr. Karen Drexler, director of the addiction psychiatry residency training program and an associate professor in Emory University's psychiatry and behavioral sciences department.
"Usually when you are sleeping, your body naturally remembers to breathe. In the case of a heroin overdose, you fall asleep and essentially your body forgets."
Studies show intravenous heroin users are 300 times more likely to die from infectious endocarditis, an infection of the surface of the heart.
Heroin use can also cause an arrhythmia -- a problem with the rate or rhythm of the heartbeat. During an arrhythmia, the heart may not be able to pump enough blood to the body, and lack of blood flow affects your brain, heart and other organs.
Heroin use can also cause pulmonary edema. That's when the heart can't pump blood to the body well. The blood can back up into your veins, taking that blood through your lungs and to the left side of the heart.
As pressure in the blood vessels increases and fluid goes into the alveoli, the air spaces in the lungs, this reduces the normal flow of oxygen through your lungs, making it hard to breathe. This too can give you a heart attack or lead to kidney failure.
Heroin can also come with other toxic contaminants that can harm a user -- although deaths from such instances, while not unheard of, are thought to be rare.
Studies suggest instantaneous death is unusual. One study showed such deaths, where a needle and syringe are still in place, account for only 14% of heroin-related deaths.
Heroin deaths increase when...
There are some common social characteristics in heroin deaths. Most fatalities involve men, particularly those who have struggled with other drugs or alcohol and other drugs or alcohol are often present.
An addict does have a much higher chance of dying if he or she leaves treatment. The risk of death is higher for newly clean heroin addicts. A number of fatalities appear to happen after periods of reduced use, one 2000 study showed.
In fact, long-term users who die from overdoses are likely to have heroin levels no higher than those who survive.
That may be in part because those who are newly clean don't know how much of the drug to give themselves any more, Drexler said. They won't need the same amount to get high as when they were using more regularly.
There are also some studies that show tolerance to the respiratory depressive effects of opiates increases at a slower rate than tolerance to the euphoric and analgesic effects. As your tolerance to the drug develops, you typically need more of it to produce the high you are used to getting. This may be why long-term users are potentially at greater risk of overdose than novices.
However, Drexler said newer users can overdose because they don't know how much drug to take, compared to experienced users. "I think it is misleading to say you would not die if you only use it once or twice," she said.
A person's chances of dying from heroin use increase dramatically after 20 years of use. Studies show that after 30 years of use, 16% of heroin users have died, compared with 6.5% of cocaine users and 1.5% of meth users.