The "best by" and expiration dates on the box of cookies or jar of peanut butter in your pantry may not seem like a big deal, but is the same true for the medications in your bathroom cabinets?

You've likely used shampoo or sunscreen days — or even months — beyond their expiration dates and that is probably OK. But it's a different story for medications, says Erin Burton, a pharmacy manager at CVS Health.

"It's important to check out the expiration dates on medications because once we're past that date, they become less effective," Burton tells CNBC Make It.

Depending on what you're treating, taking ineffective, or less effective, medicine can be harmful, she adds. "You don't want to be taking a medication that's not helping you with the disease you're trying to treat," especially if it's a severe condition.

The bottom line: "People really need to follow what's printed on the label," says Burton. "Really we shouldn't be using them a day past, but no one's perfect."

'There are certain medications that we really don't want lying around our home'

Keeping medication past its expiration date, or not disposing of leftover medicine, leaves room for them to fall into the wrong hands, Burton notes.

"There are certain medications that we really don't want lying around our home," she says, including pain medicine like opioids, stimulants or depressants. These prescriptions can be very addictive, she adds.

"We know that two-thirds of teens who misused pain relievers in the past year, say they got them from a family [member] or friend. That includes our home medicine cabinets," says Burton.

How to properly dispose of expired medications

Given the decrease in effectiveness after expiration dates, and the potential harms of keeping old medications in cabinets, you should dispose of prescriptions that you no longer need.

But, there is an ideal way to do so, that Burton strongly encourages:

• Find a take-back site near local pharmacies. "One important note is that you want to remove any personal information from the labeling before disposing of the medication," says Burton.
• Toss them in household trash. You should mix them with an undesirable substance and place them in a sealable container or bag.

CVS drug disposal units accept prescription medications, over-the-counter treatments and liquid medicine. They don't accept illegal substances, needles or syringes, medical devices or batteries, aerosol cans or inhalers.

You can throw away inhalers by contacting your local trash or recycling facility, so it can be disposed of safely, Burton says.

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