The nature and amount of press coverage is often a key factor in assessing how clinical trial results impact a product’s prescription trends.

In the case of the ARBITER 6-HALTS trial, Merck’s cholesterol franchise has benefited from largely balanced coverage, limiting damage to lipid drugs Zetia and Vytorin.

The study showed that Abbott Labs’ Niaspan, an extended-release niacin, bested ezetimibe—an ingredient in Merck’s cholesterol-lowering drugs Zetia and Vytorin—on an ultrasound measurement of artery wall thickening, which can later lead to heart attacks.

An article in The New England Journal of Medicine criticized Zetia and said its use should be restricted. Two NEJM editorials brought balance to the discussion, and here was the first coup for Merck: neither of the editorials was written by the Cleveland Clinic’s Dr. Steve Nissen, a feared critic.

The second: even Zetia’s long-standing foes seemed to take a subdued approach when discussing the data. According to Bernstein analyst Tim Anderson, in press coverage, several experts supported Zetia, saying the difference between the products may have been exaggerated.

While it was no shock that HDL raiser Niaspan won, some had feared a repeat of 2008, when the findings from the ENHANCE trial were made public. Since then Zetia scripts in the US are down by about 30%, and Vytorin—a combination of Zetia and Merck’s Zocor—is down by about 45%, according to Bernstein figures. Internationally, the drugs have still been able to grow albeit at a moderating rate. Because of this, any incremental damage from other trials like ARBITER 6 was expected to be limited.

Prescription trends show the franchise scarcely went unscathed, though. For the week ending Friday, Nov. 20—after the American Heart Association and NEJM released results of the study on Saturday, Nov. 14—patients new to Vytorin filled 5,092 prescriptions for the cholesterol therapy and 6,628 for Zetia, marking a decrease of 20% and 26%, respectively, from the week ending Nov. 13, according to SDI.

As of the week ending Dec. 11, NRx levels for Vytorin and Zetia were down 25% and 27%, respectively, compared to the week before the ARBITER data release. Despite these declines, Zetia/Vytorin are still likely to sell around $3.5 billion worldwide in 2010 (a 12% year-on-year decrease), according to Anderson, who said their financial contribution to Merck will continue to diminish.

IMPROVE-IT, a large outcomes trial expected to provide more definitive answers about the utility of Zetia/Vytorin in 2012, and the FDA’s pending analysis of the 2008 SEAS trial showing a possible cancer link with Vytorin are events ensuring that questions about the drugs will persist into the future.