The implosion of single-stock funds in South Korea that used leverage to amplify returns for memory chip leaders SK Hynix and Samsung is a sign of just how dangerous these popular products can be when volatility ramps up.

U.S. investors buying similar types of funds for top American stocks and indexes need to be extra careful as well.

South Korean regulators have already announced stricter rules to try to curb the massive price swings, which have sent the Korea Composite Stock Price Index, known as the KOSPI, tumbling. But there are new reports suggesting that even tougher regulations may be put into place.

The big price declines for SK Hynix and Samsung over the past week have hurt U.S. investors who own the new Nasdaq-listed shares of SK Hynix, as well as exchange-traded funds with big exposure to both companies, such as the Roundhill Memory ETF and iShares MSCI South Korea ETF.

But the single-stock funds, which use options and other derivatives to boost their daily returns, move even more dramatically than the underlying stocks. While SK Hynix’s U.S. shares are down nearly 20% in the past five days, the Leverage Shares 2X Long SK Hynix Daily ETF has plunged 37%.

Single-stock ETFs are designed for traders with a very short-term time horizon. The companies behind these products readily admit that they are not meant for longer-term buy and hold investors.

Still, leveraged ETFs tied to top stocks and major indexes are growing in popularity in the U.S. as well. And as was the case with SK Hynix, many of them are launching just shortly after companies go public. Several single-stock ETFs tied to SpaceX debuted in the week following its IPO.

Steve Sosnick, chief strategist with Interactive Brokers, said in a report Monday that the company sees “a love of leverage” by its customers. Five of the most 25 actively traded tickers on Interactive Brokers over the past week were for leveraged ETFs, including funds tied to Sandisk and Micron as well as the Philadelphia Semiconductor Index and Nasdaq-100.

Others are worried about the heightened risks tied to single stocks. Scott Chronert, head of U.S. equity strategy at Citi Research, said in a report Monday that “single stock volatility has been immense” under the surface for major indexes like the S&P 500.

The bottom line? Single stock ETFs are just another example of too much giddiness in a bull market that may be nearing its final stages.

“It has been a momentum freight train market,” Matthew Miskin, co-chief investment strategist with Manulife John Hancock Investments, said in an interview with Barron’s.

He said the combination of more leveraged ETFs, the increased pace of big initial public offerings this year and companies taking on debt to fund AI investments are all “signs of excessive optimism.”

Investors in South Korea learned that the hard way with leveraged ETFs tied to SK Hynix and Samsung. Their U.S. counterparts need to avoid making the same mistake with single-stock ETFs tied to chip stocks and other AI momentum darlings.