Buying skincare products in bulk can help you save money—but stocking up on products with active ingredients and short shelf lives isn't the best for your wallet or your skin.

Plenty of us have done it: You find a skincare product you love and immediately want to buy it in bulk to save money—and so you won't run out. But just because you can do that doesn't mean you should. Sure, you could cash in on multiple two-for-one deals at once, but some skincare products have shorter expiration dates—which means buying in bulk could actually end up costing you more if you end up tossing half of what you bought in a couple months.

"Two or three at a time should be fine for most skincare products," says Peterson Pierre, MD, a board-certified dermatologist in Los Angeles. "If it's a dozen, I would caution against it because ingredients do expire and you want to ensure maximum freshness and potency." Using expired products isn't that great for your skin either—at best, the product won't be as effective, and at worst, it can irritate your skin and give you contact dermatitis, according to Harshal Ranglani, MD, a clinical and aesthetic dermatologist.

Dr. Ranglani says the average shelf life of skincare products is about 18 to 24 months, and suggests using the product within a year of opening. European skincare brands that you can find in the U.S. will usually have a label that lets you know how long you can use a product after opening it. It's OK to buy certain skincare products in bulk if you happen to find them on sale—just don't open them all at once, or they'll go to waste, says Heather D. Rogers, MD, a board-certified dermatologist. "Store it some place where it is cool, and grab it when you get through your other one—and enjoy the fact you got a discount," says Dr. Rogers.

Next time you want to stock up at a skincare sale, here are the types of products that will save you money—and the ones that will waste it.

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Don't Buy in Bulk: Vitamin C & Retinoids

If a product claims to make changes to the appearance of your skin and has active ingredients in it like vitamin C or retinoids that work to target specific skin concerns, you want to watch out—because it won't have a long shelf life. "Vitamin C is a notoriously unstable ingredient that is often stabilized by adding other ingredients, such as vitamin E and ferulic acid," says Dr. Ranglani. "Even then, exposure to sunlight and heat can cause it to degrade."

Vitamin C serums also tend to be pretty expensive—so it doesn't really make sense to buy more than one at a time in case they expire or go bad if they're not stored correctly or are exposed to sunlight. Serums, in general, come in small sizes—20ml or 30ml—so it's easy to go through them quickly and buy another when you need it.

"There's always something new and fun in the treatment area of skincare, those are the areas I let my patients kind of experiment a little bit," says Dr. Rogers. But bulk-buying can put a damper on that experimentation. "If you stock up on six, and something new and exciting comes, then you get FOMO," Dr. Rogers adds. So the next time you see your favorite skincare expert rave about a new product, leave room in your collection, so you don't end up wasting money throwing away what you already have.

Don't Buy in Bulk: Acne Products

Products containing ingredients that are meant to treat acne, like salicylic acid and benzoyl peroxide, need to be used way before they expire—using them after the expiration date not only makes them ineffective, but it could cause reactions. "When it comes to facial skin, one important point to note that it is thinner and more sensitive than body skin," says Dr. Ranglani. She says the risk of irritation after using an expired product is higher on the face and neck than it is for other areas of the body like your arms or legs.

Stock Up: Sunscreen

Since sunscreens are strictly regulated by the FDA, they are safe to stock up on. Sunscreen has a shelf life of three years, which means if you're using it correctly (every single day) and in the right quantities, a normal bottle should only last you about two to three months—so feel free to get the giant size at Costco. Plus, you might need to buy a few at a time to find the right sunscreen for you. Make sure you always check the date on the back label and throw away any sunscreen that is past its expiration date—otherwise, ouch.

Stock Up: Moisturizers and Cleansers

Buying cleansers and moisturizers in bulk is safe and can help you save money, too. The ones that don't have any anti-aging ingredients can last a while if stored properly. You probably use cleansers and moisturizers in your daily skincare routine anyway, and go through them quicker, so you know they won't go to waste if you buy multiple.

Stock Up: Body Products

Body care products, such as lotions and body wash, are all safe to buy in bulk. Dr. Ranglani also recommends buying multiple shampoos and conditioners because these can be safely stored for long periods of time and the surfactants in them prevent bacteria from growing.

Bottom line: It's OK to buy certain skincare products in bulk as long as they don't have active ingredients that can degrade over time. Just make sure you store those skincare purchases somewhere cool, dry, and away from sunlight and humidity to make your products last longer—so you don't waste your money when you end up having to toss and re-buy.