Biotechnology stocks have been on a tear recently, with the S&P Biotechnology Index returning 55.1 percent in the past year, compared with 14.9 percent for the S&P 500.

But many experts think the move is overdone. "To us, shares of biotech companies find themselves entering bubble territory," Darren Pollock, portfolio manager at Cheviot Value Management, told The Wall Street Journal. "We think investors are applying too high a success rate on biotech molecules."

Major biotech companies include Gilead Sciences, Amgen and Biogen.

Some market participants compare the surge of biotech stocks to the dot.com stock bubble of the late 1990s. "If any part of the market reminds me of the go-go tech years, its biotech," Jack Ablin, chief investment officer at BMO Private Bank, told The Journal. "It's an expensive sector."

"Some of these companies are attractive," Rob Arnott, founder and chairman of Research Affiliates, a firm that has developed strategies for products with $170 billion in assets around the globe, told The Journal. "But identifying which are cheap and which are froth is awfully difficult; it's a safe bet a majority don't deserve their current multiples."

CNBC commentator Jim Cramer is worried about biotech valuations too. "This is not the moment to play with speculative, small-cap biotech stocks that are losing money and might not have any major products on the market for years to come," he said on his "Mad Money" show.

When it comes to the stock market as a whole, with the S&P 500 index less than 2 percent away from its record high, many experts say equities are poised for a correction, if not something worse.

Martin Pelletier, a portfolio manager at Canada's TriVest Wealth Counsel, cited several signs that the stock market may be running out of steam.

• "Deceleration in earnings growth. The high U.S. dollar and low oil prices are starting to take their toll on corporate profitability," he wrote in the Financial Post. Analysts predict earnings for S&P 500 companies will drop 4.8 percent in the first quarter, after rising 3.7 percent in the fourth quarter, according to FactSet.
• "An over-reliance on the Fed. We are now back to the scenario where bad news economically is considered good news, as investors want the Federal Reserve to keep interest rates at ultra-low levels," Pelletier explained. Many experts say the six-year bull market for stocks was built largely on Fed easing, but now the central bank is on its way toward raising rates.

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