When was the last time you changed your toothbrush? If you can't recall, then you may need to throw "old reliable" away today.

Oral hygiene is so much more important than most people realize, says Michelle Kelman, lecturer at UCLA School of Dentistry and an advisor at Flossy.com. 

"You have to remember that oral medicine is a part of medicine, and that's something that's forgotten. Dentistry tends to be treated like that thing on the side," Kelman tells CNBC Make It.

In severe cases, not taking care of your teeth can lead to serious health conditions like heart disease and diabetes, she notes.

And though changing your toothbrush isn't the biggest puzzle piece for stellar oral health, there can be consequences for keeping the same toothbrush for too long.

Here's everything you need to know about when you should swap out your toothbrush and why. Plus, other oral hygiene tips from Kelman.

How often should you replace your toothbrush?

You should replace your toothbrush at least every three months, according to Kelman. But for some, especially people who brush their teeth a bit aggressively, you'll need a new one even sooner, she adds.

"When in doubt, you need to look at the bristles," says Kelman. If the bristles are frayed, it's time for a new toothbrush, she says. 

It's also important for your health and the health of the people you live with to get a new toothbrush after recovering from a sickness like a cold or the flu, Kelman notes.

Here's what can happen if you don't switch out your toothbrush often enough

Using a toothbrush with frayed bristles can have multiple consequences, including:

• Not thoroughly cleaning your teeth
• Leaving plaque behind
• Allowing germs to build up and spread within your mouth

"If [toothbrushes are] not in that significant shape, they're going to lose their effectiveness," says Kelman. 

And the teeth cleaners are in their best shape when they're brand new, and in their worst condition when their bristles are frayed, she says.

Here's what else you should be doing for your teeth

Now that you've thrown away your old toothbrush, there are some other oral hygiene tips that Kelman would like you to practice:

• Flossing your teeth before you brush them
• Letting your toothbrush air-dry before putting it in a closed container to prevent bacteria build-up
• Flossing your teeth at least once a day, preferably after every meal
• Getting your teeth cleaned every three months, if you can

"Without [good] oral health, you are really compromising your lifestyle," Kelman says.

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