Celebrities can generate a large amount of publicity for health campaigns. For example, Michael J Fox's foundation has raised over $350 million for Parkinson's research and singer Sir Elton John's charity has raised more than $300 million to fight HIV/AIDs.

But their efforts are not always helpful. Sometimes the advice given by celebrities conflicts with recommendations from health professionals and research evidence and poses a public health hazard. Examples include former Playboy model Jenny McCarthy's incorrect messages about vaccines causing autism; Katie Couric's recent alarmist coverage of the HPV vaccine; and TV broadcaster Sir Michael Parkinson claiming that "if you can pee against a wall from two feet, you haven't got it [prostate cancer]."

Researchers from McMaster University in Canada looked at how celebrities gain credibility as medical advisors and why the public can fall under their influence when making important health decisions. They analyzed economic, marketing, psychology and sociology studies from 1806 to the present day.

The researchers give several explanations for how celebrities gain credibility as medical advisors.

One explanation is "herding," which is people's natural tendency to make decisions based on what others have done in similar situations.

Another explanation is celebrities' "halo effect" which, as the researchers say, gives celebrities a "cloak of generalized trustworthiness which extends well-beyond their industry or expertise." Wanting to follow in their favorite celebrities' footsteps, consumers ignore other information and instead imitate the celebrity's health choices.

Celebrities also portray themselves as having an authentic connection to the product or behavior they are promoting. So are perceived as having greater credibility than their non-celebrity counterparts, despite having less medical knowledge and experience.

Another theory explaining celebrities' influence is that consumers want to purchase "social capital" from celebrities by acquiring their products, mimicking their lifestyles and taking their medical advice. For people seeking to raise their social status, one strategy is to imitate celebrity behaviours. Furthermore, consumers have a "self esteem motive" as they follow advice from celebrities who match how they want to perceive themselves and feel like they can become more like their favorite celebrity by purchasing products they have endorsed.

The study says health professionals can counter celebrities' negative influences by speaking to their patients about the validity of celebrity advice and cement themselves as sources of reputable health information. "We need to rethink and better understand where people obtain their health information and what makes them act upon it," said Hoffman. "Understanding why people follow celebrities' medical advice represents a good start."

Editor's Note: The British Medical Journal traditionally publishes a Christmas issue containing a number of articles of a lighthearted nature. For a full list of articles in the 2013 issue, see: http://www.bmj.com/content/347/7938