According to The Washington Post, the change was made in response to outcry over online marketing by stem cell clinics touting unapproved treatments for everything from joint pain to Alzheimer's. At best, many of the clinics are a waste of money. At worst, they can be extremely dangerous.

The policy includes a list of unapproved substances that cannot be advertised, including those with ephedra, human chorionic gonadotropin (hCG) used for weight control and herbal and dietary supplements with active pharmaceutical or dangerous ingredients. It also details how health-related advertising varies from country to country.

Google does acknowledge that "good actors" are doing important research that may lead to important breakthroughs, and the company says it will still allow advertising for clinical trial research. "Digital advertising helps fuel an open internet for people all over the world -- allowing billions of people to ask questions, find answers, and discover new ideas," Google Policy Advisor Adrienne Biddings wrote in a blog post. "We know the digital ads ecosystem can only flourish if it's a place that is safe and trustworthy for users."