He explained that the issue of oral sex spreading infections such as genital herpes, gonorrhoea and chlamydia has "only really just been noticed".
Dr Greenhouse said: "Very, very few people are using condoms for oral sex.
"They don't know that you can get chlamydia and gonorrhoea in the back of your throat without trying very hard."
Dr Greenhouse advised young people - particularly those under the age of 25 and gay men - to go for regular testing at a sexual health clinic.
He said people should also "be meticulous" about condom use, including for oral sex.
The British Association for Sexual Health and HIV (BASHH) said people are passing on more infections through oral sex than previously thought.
It is believed that oral sex is now the "principal mode of transmission of drug-resistant gonorrhoea".
Dr Greenhouse added that antibiotics used to treat gonorrhoea could cure the infection in the genitals, but it is more difficult for it to penetrate the throat area, meaning it can be passed on.
A report published towards the end of last year suggested that human papilloma virus (HPV) passed through oral sex could soon overtake smoking to become the leading cause of mouth cancer among men and women.
Experts advised people to prevent the spread of HPV by using a condom during oral sex.
A latex square, known as a dam, can also be placed over a woman's genitals to protect against infection.