According to the American Osteopathic College of Dermatology (AOCD), brittle nails fall into two categories: dry and brittle or soft and brittle.

Dry and brittle nails are the result of too little moisture. They’re most commonly caused by the repeated washing and drying of fingernails.

On the other hand, soft and brittle nails are caused by too much moisture, often a result of overexposure to detergents, household cleaners, and nail polish remover.

Other causes of brittle nails include:

• Age. Nails commonly change as people age, often becoming dull and brittle. While toenails commonly get thicker and harder, fingernails often become thinner and more brittle.
• Iron deficiency. This condition occurs when the body doesn’t get enough iron, which leads to low red blood cell levels. Your doctor may measure your ferritin level and provide supplementation if it’s found to be low.
• Hypothyroidism. Along with brittle nails, symptoms of low thyroid levels may include hair loss, fatigue, weight gain, constipation, and depression. Your doctor can treat hypothyroidism with the synthetic thyroid hormone levothyroxine, which can be taken orally.
• Raynaud’s syndrome. Characterized by circulation problems in the extremities, this condition can affect nail health. Your doctor may prescribe calcium channel blockers, such as amlodipine or nifedipine, or alternatives, such as losartan, fluoxetine, or sildenafil.

The AOCD offers a diagnostic tip to help determine if your brittle nails are caused by an internal condition or external environment factors: “If the fingernails split, but the toenails are strong, then an external factor is the cause.”

You can’t do anything about age-related nail changes, but you can reduce the risk of split, cracked, and brittle nails. To keep nails healthy and strong, try the following tips:

Use a moisturizer

• Look for moisturizing hand lotions that contain lanolin or alpha-hydroxy acids. You can also buy lanolin-rich nail conditioners online.
• Moisturize your hands after washing. When applying the lotion or cream, be sure to rub it around and directly on your nails.
• Before going to bed, moisturize your hands, feet, and nails to keep them hydrated as you sleep.

Protect your hands

• When doing household chores, wear gloves, such as dishwashing gloves, to keep your hands dry. Gloves can also protect your hands and nails from harsh chemicals, such as detergents and cleaning fluids.
• Avoid prolonged exposure to cold, dry weather. If you do venture outside on a cold day, be sure to wear gloves.

Care for your nails

• Keep your nails short to minimize nail surface area, where water and chemicals can be absorbed.
• Use a fine emery board to file your nails. It’s a good idea to file your nails daily to eliminate irregularities and prevent breakage and splitting. Be sure to only file in one direction.
• Don’t pick or bite your nails or cuticles. You can use a metal instrument to push back the cuticle, but avoid using it directly on your nail.
• Buff your nails in the same direction as the nail grows. Avoid back-and-forth motion that can cause splitting.
• Consider applying a nail hardener to help strengthen nails.
• Opt for a nail polish remover that doesn’t contain acetone, and try to avoid frequent use of the remover.

Speak to a doctor

Ask your doctor about the benefits of taking a biotin supplement. According to the AOCDTrusted Source, biotin taken orally can prevent nail splitting and breakage.

A 2007 studyTrusted Source recommends a 2.5-milligram daily dose of biotin to improve nail health.

According to a 2010 studyTrusted Source, the average adult fingernail grows about 3.47 millimeters (mm) per month. Toenails grow much slower, at a rate of 1.62 mm per month.

Although these numbers vary among individuals, it generally takes about 6 months for adult fingernails to grow out completely and about 12 months for toenails to grow out.