NEW YORK (CNN/Money) - Schering-Plough Corp. said Wednesday that first-quarter and full-year profits will miss analysts' forecasts because the loss of patent protection on allergy drug Claritin continues to hurt sales.

The latest warning sent shares of Schering (SGP: Research, Estimates) down 53 cents, or 3 percent, to $16.60 for their lowest finish since Jan. 8, 1997.

Schering-Plough said it expects earnings per share to range from 75 cents to 85 cents this year. That's below the $1 per share consensus estimate of analysts surveyed by earnings tracker First Call.

Schering said the loss of Claritin will also hit its first-quarter profit, now expected to be about 10 cents a share. Analysts expected 25 cents a share, on average.

"We knew 2003 was going to be a challenge, but it appears it's going to be more of a challenge than we expected," Robert Hazlett, an analyst at SunTrust Robinson Humphrey, told Reuters.

Schering blamed the shortfall on the company's transition away from Claritin, which last year lost patent protection and now sells over the counter at much lower prices.

In 2001, Claritin enjoyed $3 billion in sales. To fill that gap, Schering has pinned its hopes on the cholesterol-lowering drug Zetia that it is marketing with Merck & Co.

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"Full-year financial results will be negatively affected by the loss of the previous profit stream from U.S. prescription sales of Claritin, occurring simultaneously with the market introduction of Zetia, whose sales are just beginning," the company said.

Zetia enters a market crowded with cholesterol-lowering drugs, including Zocor from Merck (MRK: Research, Estimates) and Lipitor from Pfizer (PFE: Research, Estimates).

The seasonal allergy market has also become more competitive. Wyeth and Johnson & Johnson are moving in to offer generic versions of Claritin. Merck this year won approval to sell its asthma drug, Singulair, as a treatment for hay fever. Aventis has a patented allergy drug, Allegra. Pfizer already sells Zyrtec.

In its warning, Schering also blamed lower-than-expected patient demand in Japan for its hepatitis C treatments.

The company, based in Kenilworth, N.J., has disappointed investors before. In January, Schering readied Wall Street for a fourth-quarter and full-year profit shortfall. Investigators are also looking into a mysterious October tumble in Schering stock that came when the company met privately with an institutional investor in Massachusetts. The company later issued a profit warning.

Schering has not been accused of violating fair disclosure rules, which mandate that companies release material information to all investors simultaneously.

Trading in Schering stock was halted ahead of the late-afternoon warning, then resumed before the trading day's end. The latest losses cut the company's stock market value to $24.4 billion. Those losses may only revive the takeover talk that Schering has been the subject of for years.