WASHINGTON (Reuters) - Two top Democratic lawmakers called on several drug companies on Tuesday to voluntarily curb advertisements targeting consumers, including withholding any ads for new products for two years.

In a letter to the chief executives of Merck & Co Inc, Pfizer Inc, Johnson & Johnson and Schering-Plough Corp, Reps. John Dingell and Bart Stupak asked the companies to also refrain from marketing products to consumers until certain studies are completed.

The Michigan Democrats are investigating whether companies are misleading potential patients through television commercials and other direct-to-consumer advertisements.

"To date, we have not received adequate assurances that the leading pharmaceutical companies share our commitment to providing consumers with accurate information about drug therapies," Dingell, head of the U.S. House of Representatives Energy and Commerce Committee, said in a statement on Tuesday.

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The letters come less than two weeks after the committee's health panel questioned lower executives from the drugmakers about aggressive marketing tactics for two cholesterol drugs -- Pfizer's Lipitor and Merck and Schering-Plough's Vytorin joint venture.

The committee also probed ads for Johnson & Johnson's anemia drug Procrit.

At the hearing, Stupak called on the companies to rein in their campaigns or face tighter restriction and possibly an advertisement ban.

Kassy McGourty, spokeswoman for Johnson & Johnson's OrthoBiotech unit, which makes Procrit, said the company received the letter and would cooperate with the committee.

Representatives for Pfizer, Merck and Schering-Plough could not be immediately reached for comment.

(Reporting by Susan Heavey; Editing by Tim Dobbyn, Gary Hill)