MEXICO CITY—Colombian President Juan Manuel Santos said the war against drugs has failed, and the world must come up with new approaches to deal with a scourge that has killed thousands of Colombians.

In an interview on Monday with The Wall Street Journal, Mr. Santos noted a softening of hard-line antidrug policies both in the U.S. and in Latin America. He said the world had to develop more "realistic and pragmatic" ways to fight drug trafficking.

...