About two in five people make health care decisions based on information from social media. This could be good news for public health organizations, who have at their fingertips a way to reach billions of users. But a message launched on social media doesn’t always follow a predictable path.

A team from the Royal College of Surgeons in Ireland set out to graph a handful of public health topics on Twitter. They focused on hashtags, words marked with the # sign that help users navigate by topic. “We wanted to see who the influencers really were,” says Richard Arnett, director of psychometrics at RCSI and a creator of the project.

Nearly 50 million tweets a year touch on cancer. In the study shown here, the RCSI team mapped 90,000 posts tagged with #BreastCancer sent over the course of eight weeks. While many were part of awareness and screening campaigns, others were hijacked for unrelated discussions.

On other divisive topics, such as vaccination or abortion, the images show conversations diverging into two or more separate camps, with messages being shared only among like-minded people. “There was an awful lot of noise and a potential for messages to get bent out of shape,” says co-creator Eric Clarke, a lecturer in health informatics.

The team hopes that these maps help demonstrate that social media outreach carries benefits and risks. “It’s only one instrument in the toolbox to reach individuals,” says Arnett. “But it is increasingly an important—and unpredictable—tool.”