We all know that a protein-packed salad is a better lunch option than a basket of fries—our bodies need the nutrients found in fresh, whole ingredients for cell regeneration and general health, after all. But, for real, it’s pretty often we promise ourselves a smoothie tomorrow in exchange for the drive-thru today—so much so that many Americans are vitamin-deficient without even realizing it.

Those deficiencies can make themselves known in sneaky ways: brittle nails, dull skin, thinner hair… and the booming dietary supplement industry promises magic pills that can refill our depleted vitamin tanks. But what are those “hair, skin, and nails supplements” composed of, anyway? And do they really make a difference?

What’s in those supplements?

These supplements are supposed to do just that: supplement the vitamins, minerals, and fatty acids we get from our diets to keep hair, skin, and nails healthy. Most supplements meant to stimulate hair and nail growth or brighten and clear skin contain some combination of biotin, fish oil, and Vitamins A, C, and E.

How do they work?

According to Michele Green, M.D., of RealSelf, the ingredients in supplements all work in conjunction with one another to provide the nutrients essential for healthy cell regeneration.

“Biotin strengthens the hair and increases its density,” Green says. “Fish oil makes hair and nails shiny and is a great anti-aging supplement, as the omega oils found in fish oil stimulate collagen production and overall appearance of the skin.”

According to Green, Vitamin A, much like its synthetic counterpart, isotretinoin, or Retin-A, can help to treat acne and reduce fine lines. “Vitamin A, taken orally, reduces sebum production, which is great if you suffer from acne. The reduction of sebum reduces acne flares, and it is also anti-aging due to its ability to stimulate cell turnover,” Green says.

“Vitamin C is an antioxidant, which helps us fight free radicals, brightens the skin, and stimulates collagen production,” Green says. “And vitamin E is an anti-inflammatory and also helps with cell turnover.”

Supplements are great—for some people.

But if supplements were such a cure-all, wouldn’t everyone be taking them? In reality, supplements have been proven to work, but only for people with health concerns that interfere with their ability to absorb nutrients.

Supplemental biotin was recently found to be effective for hair and nail regrowth in a study of 18 different cases, but all of the patients using biotin had an “underlying pathology” for damaged hair and nails. The study concluded that biotin might not be effective for people without medical conditions causing hair and nail breakage.

Another use for biotin could be clearing discolored toenails. If your toenails are yellowish for no good reason, like fungus, supplements might be the answer, according to New York podiatrist, Bruce Pinker, DPM, PC.

“When toenail biopsy results are negative for fungus, yeast, or mold (the three elements we look for in toenails that are discolored), we recommend biotin supplementation to improve the appearance of the toenails,” Pinker says. “I normally recommend 2,500 micrograms a day of biotin for about three months to improve the appearance of toenails. It’s effective and can also be taken as 5,000 micrograms a day since it’s more commonly found in this dosage.”

Recent studies have also found fish oil to be effective in treating keloid scars, while topical Vitamin A has been used for years to treat acne. And there’s some evidence to support the idea that even non-prescription topical solutions can reduce hyperpigmentation and fine lines.

But when it comes to oral supplements for healthy people who are just looking for longer nails or thicker hair, the overwhelming medical opinion seems to be a resounding, “Who knows?”

Make sure you’re getting what you pay for.

Since there’s no evidence that supplements are actively bad for hair, skin, and nails, not to mention the evidence that they’ve helped at least a few people, supplements are probably worth a shot if you’re interested in them. One word of caution: Since supplements aren’t heavily regulated by the FDA, it’s entirely possible to purchase snake oil instead of fish oil, so it’s important to be discerning when shopping for supplements.

“Shopping for vitamins can be tricky,” Green says. “You want to be sure the label says they’re 100 percent natural and list all-natural ingredients with no synthetic fillers.”

And for those who hate the idea of taking a bunch of pills every morning, Green says gummy options are a fine alternative—as long as you’re taking them before you brush. “The nutritional value of gummy vitamins includes Vitamin D, C, E, and A, along with B vitamins and iodine.

Gummy vitamins do contain gelatin and glucose syrup, but as long as you take the recommended dose, those things shouldn’t be harmful to your body. Simply make sure to brush your teeth shortly after taking them, so the sugar doesn’t sit on your teeth for long periods of time.”

And if you decide to start a supplement routine, stick with it—most people don’t see results for three to four months.

Emily Alford lives in Brooklyn, NY, and writes about beauty, food, and TV. Sometimes all at once. Follow her on Twitter @AlfordAlice.