Arnica is a genus with about 30 perennial, herbaceous species, belonging to the sunflower family (Asteraceae). The genus name Arnica may be derived from the Greek arna, "lamb," in reference to the soft, hairy leaves.

This circumboreal and montane (subalpine) genus occurs mostly in the temperate regions of western North America, while two are native to Eurasia (A. angustifolia and A. montana).

Arnica used to be included in the tribe Senecioneae because it has a flower or pappus of fine bristles. This was soon questioned and Nordenstam (1977) placed it tentatively in tribe Heliantheae s.l. This arrangement also became uncertain because of the sesquiterpene lactone chemistry in certain species. Lately Arnica was placed in an unresolved clade together with Madiinae, Eupatorieae, Heliantheae s.s. and Pectidinae.

Several species, such as Arnica montana and Arnica chamissonis, contain helenalin, a sesquiterpene lactone that is a major ingredient in anti-inflammatory preparations (used mostly for bruises).

Arnica species are used as food plants by the larvae of some Lepidoptera species including Bucculatrix arnicella.

Arnica is also known by the names Mountain Tobacco and, somewhat confusingly, Leopard's bane and Wolfsbane—two names that it shares with the entirely separate genus Aconitum.

Arnica plants have a deep-rooted, erect stem that is usually unbranched. Their downy opposite leaves are borne towards the apex of the stem. The ovoid, leathery basal leaves are arranged in a rosette.

They show large yellow or orange flowers, 6–8 cm wide with 10–15 long ray florets and numerous disc florets. The phyllaries (a bract under the flowerhead) has long spreading hairs Each phyllary is associated with a ray floret. Species of Arnica, with an involucre (a circle of bracts arranged surrounding the flower head) arranged in two rows, have only their outer phyllaries associated with ray florets. The flowers have a slight aromatic smell. If taken in the wrong dose it can be very dangerous.
The seedlike fruit has a pappus of plumose, white or pale tan bristles. The entire plant has a strong and distinct pine-sage odor when the leaves of mature plants are rubbed or bruised.

Arnica grows in the mountain woods and pastures of central Europe and Russia, the Pyrenees and in Northern America and Canada. The dried flower heads and rhizome are used herbally and was first used by Swiss mountaineers to help prevent sore and aching limbs. It has seldom been used internally because of its irritant effect on the stomach. Arnica contains the substance: helenalin which is toxic in large amounts. The roots contain derivatives of Thymol, which is one of a group of naturally occurring compounds with strong antimicrobial action (aslo known as: biocides). Thymol can even reduce bacterial resistance to common antibiotics such as penicillin.

Used on unbroken skin to alleviate sprains, muscle pains and dislocations, it is also said to be helpful when painted on unbroken chilblains. Applied promptly, it helps prevent the formation of bruises by stopping the capillaries from bleeding. It works by stimulating the activity of white blood cells which process congested blood, and by dispersing trapped fluids from joints and muscles and bumped and bruised tissue. It also has anti-inflammatory and anti-bacterial qualities and it is these that help to reduce pain and swelling as well as improving wound healing.

Recent trials involving a team of rugby players and a group of mothers with young children compared the efficacy of different bruising treatments.

A variety of bruising treatments were used including anti-oxidant injections, sports compresses, painkillers and vitamin K, which is thought to speed up healing, as well as Arnica cream and pillules.

In virtually all cases, Arnica was the favoured option: while the cream was effective in treating bruises and swelling, the pillules also treat shock and can provide relief faster as there is no need for it to be absorbed through the skin.

Arnica is often taken alongside Vitamin C and bioflavonoids, as both of these further boost the capillaries to withstand knocks.

Although Arnica may be an historical remedy, modern science has showed that the role it plays doesn't just stop with reducing bruises. Increasing evidence suggests that it also may help the body recover from surgery, toothache and even childbirth! Research on dental patients showed that arnica helped to control pain and bleeding after treatment.

Arnica montana

The species Arnica montana, native to Europe, has long been used medicinally.

Arnica montana has been used medicinally for centuries. Arnica is used in liniment and ointment preparations used for strains, sprains, and bruises. Commercial Arnica preparations are frequently used by professional athletes. According to The Memorial Sloan-Kettering Cancer Center, clinical trials "suggest benefits of arnica for osteoarthritis and reduction in postoperative swelling and pain."

Arnica is used as a topical herbal preparation as well as a homeopathic remedy. As a herbal remedy for use externally, arnica is a counter-irritant, anti-microbial, anti-inflammatory, antiseptic, anti-rheumatic, anti-neuralgic and anti-oedema medicine.

Most commonly available as a cream or ointment, arnica is used for bruises, sprains and strains, muscle aches, leg and joint aches, rheumatic pain, inflammation from sports injuries, hematomas, contusions and fracture-related oedema. Considered by herbalists as one of the primary first aid remedies, arnica is never taken internally, unless it is a homoeopathic preparation. Homoeopathic preparations are used for similar conditions, as well as pre and post surgery to prevent swelling and other side-effects.

Dosage

Creams and ointments use 20% to 25% tincture in a water or lanolin base, or a maximum of 15% arnica oil made from 1 part arnica and 5 parts vegetable oil.

Arnica contained in a rub or soothe, such as those combined with MSM or DMSO are applied directly to the skin. Dosage instructions on the product label should be followed or, a homeopath should be consulted.