Hair and nails are often stated to have much in common in relation to their origin, anatomical structures, and common involvement in many diseases. Hair and nails are predominantly epithelial structures derived from primitive epidermis and made up of keratinous fibrils embedded in a sulfur-rich matrix. It was first noted early in the 20th century that the nail unit was comparable in several respects to a hair follicle sectioned longitudinally and laid on on its side. The epithelial components of hair follicle and nail apparatus are differentiated epidermal structures that may be involved jointly in several ways as congenital and hereditary anomalies and acquired conditions such as alopecia areata, lichen planus, iatrogenic causes, and fungal infection.

Nail fungus
Onychomycosis is the most common nail disease and describes the invasion of the nail by fungi. Different clinical patterns of infection depend on the way and the extent by which fungi colonize the nail: distal subungual onychomycosis, proximal subungual onychomycosis, white superficial onychomycosis, endonyx onychomycosis and total dystropic onychomycosis. The type of nail fungus invasion depends on both the fungus responsible and on host susceptibility. Treatment of onychomycosis depends on the clinical type of the onychomycosis, the number of affected nails and the severity of nail involvement. The goals for antifungal therapy are mycological cure and a normal looking nail. Click onychomycosis for a potential natural nail fungus cure. Tea tree oil is helpful for athlete's foot.

The Food and Drug Administration is warning health care professionals not to prescribe oral ketoconazole for patients with fungal infections of the skin and nails, because of "the risks of serious liver damage, adrenal gland problems, and harmful interactions with other medicines that outweigh its benefit in treating these conditions." The advisory, issued on May 19, 2016 points out that oral ketoconazole (Nizoral) is no longer approved for treating nail or skin fungal infections. Topical forms of ketoconazole have not been associated with liver damage, adrenal problems, or drug interactions.

Nail biting
Onychophagia is a common stress-relieving habit. People bite nails in times of stress or excitement, or in times of boredom or inactivity. It can also be a learned behavior from family members. Nail-biting is the most common of the typical "nervous habits," which include thumb-sucking, nose-picking, hair-twisting or -pulling, tooth-grinding, and picking at skin. One can stop nail biting by finding ways to relieve stress.

Ingrown toe nail
Ingrowing toe nails are one of the most common foot complaints treated by a podiatrist. An ingrown toe nail can be very painful and patients may often be very reluctant to have the condition treated. An Ingrowing toe nail is caused by a splinter of nail or the whole nail causing pressure into the skin. Some Ingrown toe nails are acute which means that they have occurred due to a injury to the toe. Others are chronic, which means the patient has had the problem for a long period of time. The large toe is usually affected, however, ingrown toe nail can also affect the lesser toes.

Brittle nail cause and treatment
Brittle nails can be a normal result of aging or they can be caused by a medical condition. Genetics also is a cause since it can run in families.
  
This is a common list of reason that can cause your nails to be brittle and break easily:
Frequent use of nail polish, remover, hardeners or other cosmetic nail applications, repeated exposure to detergents or chemicals. Dehydration due to external influences such as water, detergents, or dehydrating chemicals is often a contributing or causative factor. Reduced amounts of certain vitamins, such as vitamin A, or minerals in the body, such as zinc or iron. Problems with the thyroid gland such as low thyroid.

Other causes:
Severe illness, Psoriasis, Lichen planus, Alopecia areata, Raynaud's disease, Sjogren's syndrome, Endocrine disorders, Malnutrition, Trauma, Selenium toxicity, Anemia, Fungal nail infection, Reiter's syndrome, Impaired kidney function, Psoriasis, Xeroderma, Lichen planus.

Indian J Endocrinol Metab. 2012. Mucocutaneous manifestations of acquired hypoparathyroidism: An observational study. Mucocutaneous manifestations were present in 76% of patients. The most frequent mucocutaneous manifestation was found in the hairs like the loss of axillary hair, loss of pubic hair, coarsening of body hair, and alopecia areata. The nail changes noted were brittle and ridged nail, followed by onycholysis, onychosezia, and onychomedesis.

Treatment
Patients should be instructed not to wash hands frequently and to reduce contact with water or other dehydrating chemicals. Rehydration of the nail plate, cuticle and surrounding nail fold can be obtained by soaking the nails in lukewarm water followed by application of an effective moisturizer. Alpha-hydroxy acid containing moisturizer and preparations that contain hydrophilic substances such as phospholipids have been successfully used. Occasionally, the once a week use of nail enamel is encouraged to slow water evaporation from the nail plate. It is also recommended that the patients keep their nails short, and clip them after soaking them in lukewarm water.

Supplements of iron, vitamin C and B vitamins such as biotin have been suggested to be of some value but more research is needed.

Nail pigmentation
Longitudinal pigmentation of the nail is very common. The differential diagnosis varies from subungual hematoma, to a fungal infection, to a melanocytic lesion (lentigo, nevus melanoma, etc.) to others.

Psoriatic nails
JAMA Dermatol. 2015. A Chinese Herb, Indigo Naturalis, Extracted in Oil (Lindioil) Used Topically to Treat Psoriatic Nails: A Randomized Clinical Trial.

Emails
Q. I go to a nail salon for nail care and pedicure. I heard that it is possible to catch an infection at a nail salon. Is that true?
   A. It is possible to catch a nail infection or nail fungus if the nail salon does not follow proper hygiene.

Q. I have developed vertical nail lines, or nail ridges, in my fingernails. I'm wondering if there is a vitamin, or mineral, that I am not getting enough of, or am not absorbing completely. Can you please guide me to the product that might help my finger nail cause.
   A. We are not aware of a specific vitamin or supplement that could take care of a fingernail ridge or vertical line, but you could google vertical line nail and come across some ideas.

I have been taking glucosamine for about 7 years and have noticed that my weak, brittle nails are much stronger. Every time I stop taking glucosamine, my nails weaken.