Last year, a large study suggested that the combo pill Vytorin, which combines Zetia with a statin, was no more effective than Zocor, which is a statin available as a generic.
Both Zetia and Vytorin are sold by Merck, and Niaspan is made by Abbott Labs, which sponsored the latest study.
Merck says it stands behind its drugs.
The research is published in the New England Journal of Medicine.
