Be honest: Do you actually know how old your toothbrush is? Neither do most people. Toothbrushes tend to stick around until the bristles look like bedhead—or until your dentist hands you a free replacement six months later. Turns out, both are a little too late.

So if you’ve been brushing with the same old toothbrush twice a day and are wondering why your dentist still isn’t happy with your oral hygiene, here’s what to know about swapping out that ratty old thing. And if you’re already regularly changing out your toothbrush, we have some tips on making sure you’re brushing properly and with the correct tool.

How often do you need to change your toothbrush?

In general, you should swap in a new toothbrush (or toothbrush head if you’re using an electric toothbrush) every three-to-four months. But you should think of this as the guideline rather than a hard-and-fast rule. The condition of your bristles, rather than the calendar, should ultimately dictate how often you need a replacement. “If you see them get frayed or worn down due to excessive hard brushing, I would replace them even sooner,” says Dr. Arthur Glosman, DDS, a cosmetic dentist in Beverly Hills, California. Once the bristles start splaying outward, they stop cleaning as well as they should. That’s because they can no longer sweep plaque away from your teeth and along the gumline.

One other time to make an exception to the calendar rule? After you’ve been sick. “We naturally have billions of bacteria in our oral microbiome—some good and some bad. But when we get sick, the bad bacteria are overwhelmingly present and we don’t want to reintroduce that bad bacteria into our mouth after we get better,” he says. Swapping it out after you recover is an easy precaution.

And if your toothbrush smells weird, has visible buildup, or generally looks like something you wouldn’t want touching your mouth? It’s high time for a swap.

What happens if you use an old or frayed toothbrush?

It’s not the end of the world if you’ve been using a toothbrush well past its prime. According to Dr. Chris Classi, DDS, a cosmetic dentist at Apa Aesthetic Los Angeles, doing so won’t necessarily make you sick, but it can still compromise your oral health.

“Worn bristles lose their ability to hug the contours of your teeth, leaving behind plaque and debris,” Dr. Classi says. Over time, that means more bacteria and buildup—and a higher risk of bad breath, cavities, gingivitis, and periodontal disease.

Bristles that are worn down could also lead you to brush harder, triggering more serious issues. “That increase in pressure can lead to tooth sensitivity and thinning or wearing away of the enamel, making teeth more cavity-prone and causing damage to the gums, which leads to gum and bone recession,” says Dr. Glosman.

Anyone who’s struggled with oral health issues knows they’re no walk in the park. Considering a new toothbrush costs less than your morning cold brew, it’s one of the cheaper health upgrades you can make.

More toothbrush tips

While you’re at it, here are a few other ways to make sure your brushing routine actually does its job.

Set a replacement reminder

A toothbrush unboxing isn’t exactly a memorable occasion. Set a recurring reminder for every three to four months to eliminate the guesswork.

Opt for soft bristles

You might think that a hard-bristled toothbrush is ideal to make your teeth cleaner and whiter, but Dr. Glosman says this is misguided. Instead, he says harder bristles could cause the recession or loss of gum and bone, as well as risk tooth and root exposure and sensitivity (and ensuing cavities).