Best for: Nails that just won't grow

"Nails take an average of 10 to 18 months to grow out. Biotin (aka vitamin B7) helps them grow a little bit faster," says Downie. Biotin can also help strengthen your nails, but keep in mind that it's working on the nails growing, not the nails you can see (because those are technically dead cells), so be patient with results.

Try:
Hum Nutrition Killer Nails, $20, a vegan, gluten-free supplement that you take once a day with food.

Best for: Nails that grow superslow; weak nails

According to "The Ohio Journal of Science," studies have shown that the less cysteine (an amino acid that occurs in keratin and other proteins) in your diet, the slower your nails will grow. Adding a supplement with this amino acid can help your nails grow more quickly.

Adding more vitamin B can strengthen your nails; vitamin C helps prevent pesky hangnails; vitamin E can improve blood circulation, helping nails grow faster. Both Downie and Far agree that a healthy, balanced diet is one of the best ways to keep your nails healthy. You can also take a supplement.

Try:
Phyto Phytophanére Hair and Nails Dietary Supplement, $30, which makes nails stronger (and hair shinier) with a powerhouse blend of vitamins A, B, C and E, plus omegas 3 and 6.

Nutrafol, $88, is the nutraceutical Downie recommends to all her clients. "My hair has grown over two inches in the past eight months since I have been on it. My nails are growing faster as well ... they are nice and hard and somehow appear a bit healthier," she says.

Best for: All nail types

Fish oil is chock-full of omega 3 fatty acids, which have been linked to longer, stronger nails and shinier hair (not to mention clearer skin, thanks to its anti-inflammatory benefits) -- though there is no definitive scientific research.

Try: Carlson Fish Oil, $12.90 -- Far's favorite supplement for strong, healthy nails. "It makes a world of difference for my nails and hair when taken daily."