More than seven in 10 GPs think that most of their patients lie about how much alcohol they drink.
A survey of 191 doctors, including 183 GPs, found that GPs multiply the amount of alcohol a patient says they drink by an average of 1.6.
This includes 32% of GPs who believe that patients actually drink 1.5 times more than the amount they report.
Another 46% estimate that patients drink twice the amount they admit to.
Direct Line Life Insurance, which did the survey, also found that 14% of 2,000 British adults admitted lying about how much alcohol they consume.
The reasons given were because they thought the answer was irrelevant and because they were worried their doctor would judge them.
Others said they simply did not keep track of the amount.
Professor Helen Stokes-Lampard, chairwoman of the Royal College of GPs, said: "GPs understand that it might sometimes be difficult for people to keep track of how much alcohol they drink, and that some patients might not want to disclose the amount because they're embarrassed or worried about being judged by their doctor.
"But patients should be reassured that GPs are medical professionals, highly trained to have sensitive, non-judgmental conversations about anything that might be affecting their overall health and wellbeing."
Jane Morgan, business manager at Direct Line Life Insurance, said: "Most of us enjoy a drink from time to time, but no matter how much alcohol you consume it's important to be honest with your doctor about it.
"Without all the correct information about your lifestyle you may not get the right diagnosis or treatment."
Helen Clark, deputy director of drugs, alcohol and tobacco at Public Health England, added: "It's all too easy for the amount of alcohol you drink to creep up on you.
"That's why it's important people try to keep track of how much they're drinking."