What is henna?
Henna (hina) is a flowering plant with the botanical name lawsonia inermis. It contains a red-orange pigment, lawsone, also known as hennotannic acid. This pigment is released by crushing the henna leaves in an acidic medium. The lawsome molecules will then have the ability to stain, and when applied to the skin they migrate to the outermost layer or cuticle, and stain them.
Commercially available henna powder is made by drying the henna leaves and milling them to a powder, which is then sifted.
To use henna, the powder is mixed with water and/or lemon juice, to form a smooth paste. The henna mix must rest for 6 to 12 hours so that the leaf cellulose is dissolved, making the lawsone available to stain the skin. The longer the paste is left on the skin or hair, the more the lawsone will migrate, thus making the staining darker.
Health Benefits of Henna
Various studies have researched into the benefits of the henna plant on health. One study found that henna has natural antimicrobial properties, such as being antibacterial and antiviral. 
“Henna has a wide spectrum of antimicrobial activity including antibacterial, antiviral, antimycotic and antiparasitic activities. With the ever increasing resistant strains of microorganisms to the already available and synthesised antibiotics, the naturally available Laswonia inermis (henna) could be a potential alternative.”
Cooling Agent: Henna is known to be an excellent cooling agent and hence it is applied to scrapes and burns. Henna is also used as a home remedy to lower the body temperature while suffering from a high fever or to treat heat exhaustion.
Skin Problems: The medicinal uses of henna also include it being used to treat various types of rashes such as ringworm as well as athletes foot and also acts as a sunblock. Since the leaves of the henna plant are astringent in nature they are also used against various skin diseases.
Henna for Hair: One of the key benefits of henna for hair is that it acts as a natural remedy for hair loss and is also excellent for maintaining overall hair quality. This is because the regular use of henna is known to seal and repair the hair cuticle which in turn prevents the breakage of hair and also helps in retaining the shine of the hair. Another one of the benefits of henna for hair is that its prevents premature hair fall as it helps in retaining the pH balance of the scalp and also acts as an effective natural cure against dryness, dandruff as well as premature graying of hair. Henna is also used for hair conditioning as its makes the hair silky and soft and nourishes them right from the root. In fact it is recommended that in order to maximize the benefit of henna for hair you should soak the henna in some fresh curds and then apply this mixture on your head. This serves a dual purpose as curd ensures that the hair does not dry out and it adds to the shine of the hair.
Henna for Nails: Henna is also known to be effective in treating cracking nails. This is possible via a home remedy of drinking the water in which the henna leaves have been soaked in overnight. This treatment should continue for at least 10 days for best results. The medicinal uses of henna for nails also includes mixing together some butter and henna powder and using it as a poultice to treat pus filled swellings, mange and scabies.
Liver disorder: Henna is considered to be a highly beneficial medicinal plant and hence the bark of the henna plant is known to be effective in the treatment of the enlargement of the liver as well as jaundice.
Headaches: Henna is also known to be effective against headaches that may have been caused on account of heat strokes. One of the home remedies for headaches involving henna involves crushing the whitish, pinkish flowers of the henna plant along with some vinegar and then applying this paste on your temples to get quick relief from a headache.
Besides the leaves the other beneficial parts of the henna plant are the bark, flowers as well as the seeds of the henna plant.
 O.A. Habbal, A.A. Al-Jabri, A.G. El-Hag. (2007) antimicrobial properties of Lawsonia inermis: a review. Australian Journal of Medical Herbalism.