People like to read personal testimonies, the study says
Researchers found people use an initial weeding out process to deal with the minefield of health information of variable quality available.
However, this tends to mean they quickly eliminate most NHS and drug company websites, they said.
The study was funded by the UK's Economic and Social Research Council.
The researchers examined the internet search strategies of people who wanted to find specific health information on topics such as high blood pressure, the menopause and hormone replacement therapy (HRT).
They found that many websites were dismissed at quite amazing speeds.
Lead researcher Professor Pamela Briggs, from Northumbria University, said: "One thing that really put people off was advertising, so people clicked off drug company websites straight away.
"Generally, the medical information on drug company sites is very accurate but people question the authors' motivation and agenda.
"The issue of impartiality is quite crucial in building trust."
NHS websites fared little better. Often these were rejected because the first page participants were directed to was a portal or they had too much background or generic content.
Professor Briggs said: "People don't have the patience to scroll through pages in order to find something useful.
"Ease of access is so important."
The research also found that even if a site made a favourable first impression, it was unlikely to keep the attention if it did not include personal stories to which the reader could relate.
Many were specifically drawn to sites where they could read about the experiences of other people who have the same problems and concerns.
But Professor Briggs warned that the tendency to trust sites with personal testimonies from like-minded peers was potentially flawed, as it could reinforce unhealthy behaviour patterns.
Dr Paul Cundy, a GP and member of the British Medical Association's IT committee, said there was also a danger that personal testimonies were planted by drugs companies.
He said drugs companies had been known to sponsor self-help sites which appeared at first glance to be independent."People should be encouraged to gather information from the web, but they should then take it to a doctor to discuss face-to-face rather than immediately taking action based on what they have found," he said.