People who delay going to their GP face needing stronger treatments and too many rely on the internet for information, experts have warned.
A poll for The Information Standard - an organisation backed by NHS England - found 41% of people have put off visiting their doctor despite worrying symptoms, with more than two thirds (68%) saying this led to their problem getting worse or not going away.
More than one in three (36%) were told by their GP they should have visited earlier, one in five (20%) needed a stronger course of treatment and 17% were told they had had a lucky escape.
More than half (55%) of those who put off a trip to the doctor thought symptoms would clear up on their own and 39% did not want to waste their GP's time.
Some 53% of those who put off going to the doctor had also researched their symptoms on the internet, while others tried to self-treat at home.
When delaying, women were more likely than men to turn to the internet for health information (59% compared with 43%).
Ann Robinson, director of public awareness for The Information Standard, which certifies health and care organisations as reliable sources of information, said: "People are delaying their GP visit for various reasons but, regardless of the reason why, our concern is that people are admitting to self-diagnosing and self-treating in the meantime.
"Unreliable health information in these circumstances could then have a detrimental effect on their health.
"If people are looking for health information, they should make sure it's trustworthy so that they can make a fully informed and safe decision.
"Our advice is simple; look out for The Information Standard quality mark on health websites and leaflets - if you can see the mark then you can feel confident that the information you're reading is reliable."
The poll was of 1,500 people.