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Vitamin C is a popular ingredient in skin care products, thanks to its potent antioxidant qualities. If you’ve priced vitamin C serums at Sephora or Whole Foods lately, you might be tempted to search the internet for cheaper alternatives.

You’ll see plenty of Pinterest posts and DIY recipes on blogs that recommend using lemon juice on your face for everything from fine lines and wrinkles to whiteheads and sunspots.

Fans say dabbing a little lemon juice on a pimple will dry it right up and wiping your face with a lemon slice will brighten your skin.

Lemons cost less than a dollar. Vitamin C serums cost lots (and lots) of dollars. Sounds like an easy choice, right?

But, hold up.

Nature is powerful and substances that are A-OK to eat or drink aren’t always safe to put directly on your skin.

“Be cautious in trying internet-given home remedies,” cautions Dr. Adam Mamelak, an Austin-based dermatologist.

Keep reading to find out why you need to get your skin care advice from a pro — and why you might want to save lemons for your water.

First up, why would you want to put lemon juice on your skin?

“Lemon juice contains a high amount of vitamin C,” which does offer nutrition for your skin, says Dr. Michele Green, a cosmetic dermatologist based in Manhattan.

According to Green, lemon juice can:

  • help treat acne by reducing inflammation and oil production
  • help reduce blackheads from forming by breaking down the accumulation of dead skin cells
  • help lessen discoloration and scarring

Be aware: There’s a difference between the lemon juice in your favorite beauty product and swiping a wedge across your T-zone.

Your vitamin C serum or peel might list lemon juice as an ingredient, especially if it’s a natural or organic brand. But there’s a ton of chemistry behind the products you buy, and that bottle has been formulated to have a consistent, specific, and safe amount of lemon juice.

The same can’t be said for a lemon you buy at the bodega. Nature isn’t a factory, so you could buy two lemons at the same time that are far different in taste and acidity.

“The active ingredient in lemon juice is ascorbic acid, aka vitamin C,” explains Dr. Susan Bard, a dermatologist based in Brooklyn. While “vitamin C is a potent antioxidant and can help lighten pigmentation, it is also an acid that can burn the skin,” she warns.

She doesn’t recommend using straight lemon juice on your skin. Instead, opt for “a vitamin C serum with a known concentration of vitamin C. It will provide safer, more consistent results.”

Glad you asked. The most common reaction to using lemon juice is skin irritation. It could be mild, but it “can cause severe irritation, especially for those patients that have sensitive skin,” warns Green. (She doesn’t recommend any fruit acids if you have sensitive skin.)

Other common symptoms include:

  • redness
  • dryness
  • peeling

That’s not even the worst of it. You could end up like Samantha from “Sex and the City” after an overzealous chemical peel.

Here are the two scariest complications:

Phytophotodermatitis

Just because something is natural doesn’t mean it’s harmless. Mix citrus juice with sunlight, and you could end up with margarita burn — officially known as phytophotodermatitis.

Take a minute to look at these photos. You could end up with blisters, welts, burns and a ton of pain (and even scarring).

That’s what can happen if you get citrus juice on your skin when you’re in the sun. It’s called margarita burn because you can get it from drinking margaritas (or beer with lime wedges) on the beach.

You can also get it simply from squeezing a lemon into your water while sitting on a patio — though that’s less likely to happen.

“Citrus substances can cause an inflammatory/allergic reaction when exposed to UV rays,” says Green. “There’s also the risk of developing a sunburn with lemon juice — this can happen if you apply lemon right before going outside into the sun.”

Read the label on your vitamin C serum. It’ll likely remind you that SPF is a must after use!

Chemical leukoderma

Diluting lemon juice might reduce your risk, but it can still cause major issues. Homemade toner made with lemon juice, alcohol, and glycerin landed one woman in a medical journal.

She whipped up the beauty product to treat freckles and dark spots on her face, but she ended up with chemical leukoderma.

Caused by repeated exposure to certain chemical compounds — including those in lemon juice — this condition left her with uneven white spots that can be permanent. Yikes.

Mamelak recommends you stick with lemon in your food and drinks. The antioxidants in lemon and other fruits and vegetables “protect your skin against free radicals and sun damage, which help keep your skin looking young and radiant.”

Go ahead and use skin care products that contain vitamin C and lemon, but don’t expect miracles. Vitamin C is water-soluble, meaning you pee out what you don’t need. If you have adequate levels in your blood stream, applying it topically won’t increase your skin’s vitamin C content.

If you do need a home remedy for a pimple, Green recommends aloe and tea tree oil, which “provide anti-inflammatory and antimicrobial properties for the skin,” she says.

Beyond that, to be on the safe side, “seek care from a dermatologist when trying to treat specific skin conditions,” she warns. “They are well aware of which treatments are necessary for your skin” — and which you should skip.

How to use lemon juice safely

OK, so you still want to try it? You’re so brave. Green offers a few warnings:

  • Start with a spot treatment. Test lemon juice on a less-sensitive area, like your elbow. (Don’t start with your face!) “This ensures that you don’t develop any side effects from the acidity of the lemon,” she said. “You can start using it once daily and work your way up depending on how well your skin does.”
  • Always, always wear your SPF, especially if you’re using vitamin C or lemon juice.