Facebook Inc. and YouTube are being flooded with scientifically dubious and potentially harmful information about alternative cancer treatments, which sometimes gets viewed millions of times, a Wall Street Journal examination found.

Now, the companies say they are taking steps to curb such accounts. Facebook last month changed its News Feed algorithms to reduce promotion of posts promising miracle cures or flogging health services, a move that will reduce the number of times they pop up in user feeds, the company says. Some...