It’s easy to understand why cheap prescription drugs bought off the Internet might be risky, but phone chargers? As it turns out, buying a counterfeit phone charger is dangerous way to save a few bucks.

In recent years, stories have emerged of people in various places around the globe being hurt while using cheap, knock-off chargers on their iPhones. According to reports, one man ended up in a coma after getting a shock possibly related to using a non-Apple charger for his iPhone, and a woman was killed answering a phone that was plugged in at the time.

Related: 4 Places Never to Keep Your Cell Phone

John Drengenberg, consumer safety director for UL, warns owners of any electronic device to be very careful when purchasing third-party accessories — and especially chargers. “What you are charging is a lithium-ion battery,” Drengenberg says. “If you charge it at the wrong current it could heat up the internal phone batteries or the charger itself and cause a fire or an explosion.”

Cheap chargers have proliferated on the market, showing up as impulse buys at cash registers, for instance. “Generally what we see is high volume, low-cost items are counterfeited,” Drengenberg points out. “The consumer sees it and thinks ‘Hey, I could use another one of those and it’s only $3.’” Most don’t understand the risks.

UL takes counterfeiting very seriously as a safety issue. The company works with and trains thousands of agents with the FBI, Royal Canadian Mounted Police, Interpol and U.S. Customs and the Border Protection. Confiscated counterfeits are destroyed to avoid the chance they’ll end up in poor countries with lax customs, Drengenberg says.

UL authorizes the use of 22 billion of its marks on products each year (including legitimate iPhone chargers), signaling to consumers which manufacturers take safety seriously. UL inspectors are empowered to make unannounced visits to plants to ensure that standards are maintained over time, Drengenberg says.

Of course, counterfeiters don’t use UL to certify their products, so you won’t find the UL mark on knock-off chargers — unless the mark itself is fake. Drengenberg says he has seen at least one product with a fake UL certification, but the counterfeiter misspelled the mark as “VL.” UL is fighting back, making the marks (logos) on some products holographic, the way paper money is verified.

Considering buying a second charger for you device online or at a retail outlet? Drengenberg offers these safety tips.

Related: How to Keep Apps from Learning Too Much About You

Stick to name stores. If you shop at retailers you know and trust, you are much less likely to come home with a knock-off product. The buyers for big chains are careful about sourcing and, while not perfect, they generally do quality checks before retailing a product.

Watch out for weird English. You might not be able to detect that the amount of copper in the cord is insufficient for the current being carried, but you can spot misspelled words, missing punctuation and stilted, bad translations. Counterfeiters are not concerned at all about quality, and that sometimes shows up in language on the packaging.

No brand name? Steer clear. Avoid products with minimal packaging, no branding and no documentation. Responsible manufacturers want to build trust and attach that trust to their brand. A barrel of $1 cords with no labels is a flashing warning sign.

Don’t borrow chargers. You’re on a trip and a friend has a similar phone with the same charger jack. Unless you are certain that the charger will safely work on your phone, don’t risk it. Consult your phone’s documentation or ask your carrier or retailer to be sure.

Finally, a charger you buy to charge your phone in the car or as a second charger for home might not be counterfeit, but it could easily be the wrong one for your product and thus a real danger, says Drengenberg.

“Make sure the charger you purchase as a replacement is recommended by the manufacturer and is compatible with the product,” he says. “The jack might fit but have no relationship to the product.” In other words, it could be dangerous.

Related: What to Do If You Drop Your Laptop or Dump Coffee on Your Keyboard

Greg writes about personal finance, business and technology. His work has appeared in Businessweek, Newsweek, Forbes, Bankrate and a variety of trade ​publications.