Clinical staff are being urged to remind young patients that they must look to official sources when worried about sexual health problems.
Government-backed scheme the Information Standard, which aims to improve the reliability of health information, has warned that a survey showed eight in 10 young people questioned relied on the internet for help without being aware of the authenticity of the sites.
Of the 1,200 people aged 18 to 24 years questioned, nearly two-thirds said they had never had a sexual health screening.
The Information Standard said many are reluctant to use NHS services due to embarrassment or a lack of awareness about the seriousness of conditions.
The scheme said it was important to use reliable sites, such as those provided by the NHS, local authorities or leading charities. Official sites carry the Information Standard brand.
Director of public awareness Ann Robinson said: “We want all young people to understand how to spot a trustworthy site. Just because it ranks high in the search engine, for example, does not mean that it will provide informed advice.”