Drug overdose deaths are skyrocketing — up 10 percent in 2017 alone — and close to half the deaths are caused by synthetic opioids such as fentanyl.

These lab-made drugs can be very potent and they are increasingly showing up in supplies of drugs that buyers believed were heroin. Because the fentanyl and related drugs are so powerful, it’s easy to overdose. That’s why so many users die.

In fact, deaths from synthetic opioids now outpace deaths from heroin or other opioid drugs in the U.S., according to federal data.

But why cut heroin with fentanyl in the first place? Researchers in California say there are two reasons. “It’s cheaper than heroin, and it’s smaller, lighter and easier to smuggle,” said Sarah Mars, who studies drug addiction and policy at the University of California San Francisco.

Mars and colleagues have been interviewing drug users across the country, along with some street-level dealers who also use their own products. They also delved into the research being done on drug prices and distribution patterns to answer questions about why fentanyl and fentanyl-like drugs are becoming so common.

There’s a shortage of heroin and a growing supply of cheap fentanyl from China and Mexico, Mars found. “There is demand for fentanyl but it can’t drive the market,” Mars told NBC News.

That’s because it’s impossible for buyers to tell ahead of time whether they are actually getting fentanyl. And the researchers also found that many opioid users are actively afraid of fentanyl and don’t want it, because they know about the danger of overdose.

“Whether or not they prefer fentanyl, users don’t have any influence over what drugs are being sold,” Mars said. “Without accurate information about these drugs, they can’t make an informed choice about what they are buying."

There's another indication that demand is not driving the increased use of fentanyl. There are hardly any street slang words for fentanyl, Mars said.

Instead, the explanation seems to be that synthetic opioids are cheap and available. They are being added to the supply at the distributor level, not by dealers who sell directly to users, Mars and colleagues wrote in a report in the journal Addiction.

New fentanyl-like synthetics have hit the market and they are being found more widely. The new drugs include carfentanil, which is 100 times more potent than fentanyl, the Centers for Disease Control and Prevention says. Another is 3-methylfentanyl, which is four times as powerful. Other illicit synthetic opioids include furanylfentanyl and acrylfentanyl.

People are not usually asking for these newer synthetics, but they are being used to cut or outright substituted for products being sold on the street as oxycontin, or even as the anti-anxiety drug Xanax, Mars said.

Even methamphetamine is being cut with synthetic opioids, she said. "That one is a real puzzle. We don’t know why dealers would add fentanyl to those drugs. It doesn’t add to their potency,” Mars said.

The U.S. government has been asking China to control the production and export of fentanyl, and last Saturday China agreed to label fentanyl as a controlled substance in a step that is meant to make it easier to prosecute people for making and selling illicit fentanyl.

Mars said that might help if China actually uses the designation to reduce exports. “If it were possible to cut the supply from China, that would certainly have an impact on the number of fentanyl overdose deaths,” she said.

“Punishing low-level dealers will have no effect. They don’t know what they are selling,” she said.