BOSTON (Reuters) - Fidelity Investments’ bet on a privately held maker of e-cigarettes faces new risks after U.S. health regulators on Wednesday said they are considering a ban on flavored versions of the popular “vaping” products.

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Closely held e-cigarette maker Juul Labs and other Silicon Valley start-ups lately have turned up as side bets in portfolios from Fidelity and rivals like T. Rowe Price, as the asset managers look for big paydays once the small companies go public, as Facebook Inc did in 2012.

The U.S. Food and Drug Administration gave the five top-selling e-cigarette brands 60 days to provide plans for keeping their products from being used my minors. Juul, which owns about a 72 percent market share, was targeted by the agency for selling flavors that appear to particularly appeal to minors.

At Boston-based Fidelity, Juul is held in funds including Blue Chip Growth Fund and Fidelity Advisor Series Growth Opportunities Fund. At the latter, a stake in Juul was the largest single holding as of July 31, with a valuation of $45.8 million, or 6.4 percent of net assets, according to a current disclosure on the fund firm’s website.

Fidelity spokeswoman Meghan French declined to comment or make portfolio managers available to comment. But even managers who are fans of Juul have made clear that uncertainty remains.

“The company (Juul) and the industry face a number of social and public policy concerns,” wrote Sonu Kalra, manager of Fidelity Blue Chip Growth Fund, in a quarterly document for investors dated June 30.

However, he and others were mostly bullish on Juul. “A small private investment in Juul Labs also paid off. We saw the value of our non-benchmark position in this leading e-cigarette maker rise along with the growing popularity of tobacco-free smoking,” wrote Growth Opportunities manager Kyle Weaver in the fund’s annual report dated Nov. 30, 2017.

Other non-public companies in the Blue Chip Growth portfolio include ride-hailing company Uber Technologies Inc and Blue Bottle Coffee Inc.

Uber’s chief executive said earlier this month that the company is on track to go public next year. But the company also shows the risks of investing in start-ups not yet tested by public markets, as it faces regulatory scrutiny over several issues, including alleged violations of bribery laws and gender discrimination.

Uber has said it is cooperating with a U.S. Justice Department investigation into possible violations of bribery law. Uber CEO Dara Khosrowshahi in remarks to employees a year ago promised to change its culture after his predecessor Travis Kalanick resigned.

John Bonnanzio, editor of the Fidelity Monitor & Insight newsletter for investors, said that whatever the upside, owning Juul represent business risks, underscored by the FDA action and the difficulty of measuring the impact on a company whose shares are not publicly traded.

Pressure from fund investors could lead Fidelity managers to pull back and decide “this thing isn’t worth the trouble,” he said.