San Francisco ? Google Inc. stopped taking ads from unlicensed pharmacies, joining other popular Web sites that have bowed to pressure to curb the illegal distribution of prescription drugs such as the painkiller Vicodin.

The step follows similar action last month by Overture Services, which distributes ads to the Web sites operated by its parent company, Yahoo! Inc., and Microsoft’s MSN. America Online began restricting ad sales to unlicensed pharmacies two years ago.

Google’s ad ban doesn’t affect the results that its search engine displays.

“Google supports end user choice in commerce and we will work hard to ensure there is always consumer choice and value in online pharmacies,” the Mountain View, Calif.-based company said.

The crackdown on unlicensed pharmacies comes as regulators and Congress intensify their focus on third parties — Web sites, credit companies and shipping companies — that make it easier for illicit operators to sell potentially dangerous drugs.