Oppenheimer & Co. started coverage on ViroPharma Inc. Friday with a “Perform” rating, taking a cautious position on the company over concern that competitors for its core products are closer to reaching the market.

The stock fell 43 cents, or 4.1 percent, to $9.98 in afternoon trading. Shares have traded between $7.94 and $15.16 over the past 52 weeks.

Oppenheimer analyst John Newman said the stock is fairly valued and competitive challenges could offset any upside to sales potential for Maribavir, which is being developed to treat viral infections obtained during transplant procedures. The company’s key drug, the treatment for gastrointestinal infection Vancocin, could face generic competition.

Meanwhile, Vancocin may also lose market share to a drug being developed by Optimer Pharmaceuticals Inc.

Earlier in November, San Diego-based Optimer said its drug candidate OPT-80 was slightly more effective than Vancocin in a late-stage study of patients with gastrointestinal infections. Patients on OPT-80 also relapsed less than their counterparts on Vancocin.

Both drugs would compete in treating CDI, or clostridium difficile infection, which affects the colon. It is most commonly obtained by patients in hospitals following treatment with antibiotics. Still, OPT-80 is years away from reaching the market.

Meanwhile, ViroPharma’s recently acquired drug Cinryze could face multiple competitors in 2009. The company obtained the drug when it bought Lev Pharmaceuticals. It treats a hereditary protein deficiency that can lead to dangerous swelling.

Cowen and Co. analyst Rachel McMinn reaffirmed a “Neutral” rating on ViroPharma, but brushed off some concerns over Cinryze’s competition, including Dyax’s DX-88. That drug is under priority review by the Food and Drug Administration.

Despite the cautious outlooks from Oppenheimer and Cowen, Wall Street mostly holds a positive view of ViroPharma.

Credit Suisse, Lazard Capital Markets, Rodman & Renshaw, Piper Jaffray and Thomas Weisel all maintain “Buy” or equivalent ratings on the stock.