Health information online is breeding a generation of cyberchondriacs - people who needlessly fear the worst diagnosis after surfing the net, say researchers.
A team at Microsoft studied health-related Web searches on popular search engines and surveyed 515 employees about their health-related searching.
Web searches had the potential to escalate fears - like a headache was caused by a brain tumour, for example.
Experts said people concerned about their health should see a doctor.
Self-diagnosis by search engine
Microsoft conducted the study to improve its own search engine.
Roughly 2% of all the Web queries were health-related, and about 250,000 users, or a quarter of the sample, engaged in a least one medical search during the study.
The Web can be a useful tool to find out more information about conditions, but it should not replace talking to an expert
A spokeswoman from NHS Direct
The researchers found Web searches for common symptoms such as headache and chest pain were just as likely or more likely to lead people to pages describing serious conditions as benign ones, even though the serious illnesses are much more rare.
Searching for "chest pain" or "muscle twitches" returned terrifying results with the same frequency as less serious ailments, even though the chances of having a heart attack or a fatal neurodegenerative condition is far lower than having simpleindigestion or muscle strain, for example.
About a third of the 515 Microsoft employees who answered a survey on their medical search habits "escalated" their follow-up searches to explore serious, rarer illnesses.
Although the work does not give firm proof that searching the web increases health fears - users may simply be curious about a condition - the researchers say it is likely in some circumstances.
"Our results show that Web search engines have the potential to escalate medical concerns," said Eric Horvitz, an artificial intelligence researcher for Microsoft.
A spokeswoman from NHS Direct said health information on the Web was no substitute for expert advice.
"It is always a good idea to talk to a clinician who can point you in the right direction if you are concerned about your health.
"The Web can be a useful tool to find out more information about conditions, but it should not replace talking to an expert."
Henry Scowcroft, of Cancer Research UK, said trusted patient information websites could be useful resources.
"Paradoxically, the problem in the UK is that many people are still unaware of the symptoms of cancer, and delay in seeing a doctor is one of the key reasons why this country's cancer survival figures lag behind the best in Europe.
"It's important to study this area further, but we must also remember that many people still have no access to the wealth of information online, and that health inequalities - including inequality of information access - are widening, not narrowing."