Merck & Co's recently approved Victrelis treatment for hepatitis C considerably lessens the effectiveness of some widely used medicines against the virus that causes AIDS, Merck and U.S. regulators said in separate reports.

"These drug interactions may be clinically significant for patients infected with both chronic hepatitis C virus and HIV by potentially reducing the effectiveness of these medicines when co-administered," Merck said in a February 6 letter to healthcare professionals.

Victrelis, approved last May, attacks the hepatitis C virus that over decades can lead to cirrhosis and liver failure. A significant percentage of hepatitis patients are also infected with the human immunodeficiency virus, or HIV, which weakens the immune system and is fatal without treatment.

The drug interactions were seen in a study among healthy volunteers who took Victrelis and the widely used HIV treatment Norvir in combination with one of three other anti-HIV pills: Reyataz (atazanavir), Prezista (darunavir) and Kaletra (lopinavir/ritonavir). All of the HIV drugs work by blocking protease, an enzyme the virus requires to replicate.

Victrelis reduced concentrations in the blood of Reyataz, Prezista and Kaletra by an average 49 percent, 59 percent and 43 percent, respectively.

Further, levels of Victrelis itself were reduced by 45 percent among volunteers who took it with Kaletra, and 32 percent among those who took it with a combination of Norvir and Prezista.

ISI Group analyst Mark Schoenebaum said 10 percent to 15 percent of patients with hepatitis C are co-infected with HIV, and the findings could crimp Victrelis sales by as much as 25 percent. But he said the setback would have little impact on Merck's earnings this year or in 2013.

The reduced prospects for Victrelis come even as its sales are being dwarfed by Vertex Pharmaceuticals Inc's Incivek, a rival protease inhibitor that was also approved last May.

The U.S. Food and Drug Administration, in an announcement of the findings that appeared on the agency's website on Wednesday, said patients should not stop taking any of their medicines without talking to healthcare professionals.

Drug interactions had previously been found between Victrelis and another HIV treatment called Sustiva (efavirenz). Sustiva belongs to a family of HIV drugs called non-nucleoside reverse transcriptase inhibitors (NNRTIs).

Merck said it was conducting drug-interaction studies of Victrelis with other HIV drugs. They include Intelence (etravirine), which is also a NNRTI, and Isentress (raltegravir), which belongs to a class of drugs called HIV integrase inhibitors,