And sharing brushes or letting them touch other ones is just as bad for you, according to London's Young Dentist of the Year 2016.
She told MailOnline: "The mouth harbours literally hundreds of different types of microorganisms, some of which can be transferred to a toothbrush during use.
"Microorganisms in the bathroom or household environment may also collect on a stored toothbrush - especially after the toilet has been flushed."
"When toothbrushes are stored in close together and touching in a tooth mug, the germs can pass from one to another, leading to the risk of cross contamination.
"Rinsing a toothbrush under running water from the tap does not remove all these germs."
If you borrow a toothbrush which has been used by someone else the viruses which cause conditions such as herpes and hepatitis A, B and C can be transferred to you, as studies have shown that they can survive for months on a toothbrush.
Toothbrushes are a known source of contamination. Since the 1920s scientists suspected that the re-use of toothbrushes could be a possible source of infection in the oral cavity.
While nearly 40 years ago, a study published in the Scandinavian Journal of Dental Research discovered they also often contain Streptococcus mutans.
This bacteria is known to build-up in the mouth, being a leading cause of tooth decay and the breakdown of enamel - the thin protective covering.
The same scientists also argued that using the same toothbrush helped to kill off healthy germs found within the mouth.
Instead they can introduce more dangerous ones to the mouth, including enteric bacteria and pseudomonads.
These can lead to diarrhoea, skin rashes and ear infections, among other things, according to medical literature.
And in 2015, researchers from Quinnipiac University in Connecticut found that 60 per cent of toothbrushes are covered in faeces.
But they also discovered that there's only a 20 per cent chance of it being your own - instead it's more than likely to be your husband's, wife's, or whoever your share your house with.
Another study led by Professor Richard Glass in 1988 found that the Herpes virus can remain viable on a dried toothbrush for at least 48 hours.
While in a moist environment, such as that of a bathroom, it can survive for longer than a week, according to the University of Arizona scientist.
WHAT YOU CAN DO
TV dentist Dr Mervyn Druian - recommends the following:
1. Don't flush where you brush
If you must store your toothbrush near your toilet make sure you close the lid when you flush to minimise the dispersal of contaminated water droplets, and encourage your family to do the same.
2. Store it upright and alone
Store your toothbrush in an upright position - never in a bathroom cabinet where air cannot circulate as this could promote bacterial growth. Try and avoid putting more than one brush in a tooth mug to avoid cross contamination.
3. Replace your toothbrush regularly
Make sure you replace your toothbrush and Steripod every 3 months - buy them at the same time. And if you've been suffering with a cold or flu it's a good idea to replace your toothbrush.
4. Don't share it
This should go without saying, but never be tempted to share a toothbrush, even with your loved ones.
