The Food and Drug Administration was dragged into the online world of medical misinformation this week, telling consumers not to drink bleach solutions that are being marketed as cures for autism, cancer, H.I.V./AIDS and other medical conditions.

It was the latest example of how health authorities must sometimes pit science against the viral power of the internet, which regularly serves as a platform for inaccurate medical advice and unproven claims of breakthroughs.

The F.D.A. has previously taken a stand against the “cruel deception” of supposed cures for cancer, the “dangerous scam” of bee pollen products for weight loss and claims that a dietary supplement can treat a concussion (“No!”).

In a statement on Monday, the F.D.A. said it first told consumers not to drink the bleach products in 2010. But its hand was forced again after the products continued to be promoted on social media and sold online by independent distributors, and the agency recently received new reports of people being sickened.