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

They 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. The flowers have a slight aromatic smell. The seed-like 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 montana has been used medicinally for centuries. The roots contain derivatives of thymol, which are used as fungicides and preservatives and may have some anti-inflammatory effect. Arnica is currently used in liniment and ointment preparations used for strains, sprains, and bruises. Commercial arnica preparations are frequently used by professional athletes. Arnica should not be taken internally due to its toxicity, with the posible exception of homeopathic preparations that are diluted at 24X or more, since only water remains.

Arnica contains the toxin helenalin, which can be poisonous if large amounts of the plant are eaten, and contact with the plant can also cause skin irritation. If enough of the material is ingested, the toxin helenalin produces severe gastroenteritis, and internal bleeding of the digestive tract.

Arnica preparations used topically have been demonstrated to act as an anti-inflammatory and assist normal healing processes by facilitating transport of blood and fluid accumulations through a dilating action of subcutaneous blood capillaries

Homeopathic preparations of Arnica are widely marketed and used, and enjoy popularity. In the UK the Medicines and Healthcare products Regulatory Agency has registered the product for sprains and bruising under the 'National Rules for Homoeopathic Products' (2006). These rules allow claims of efficacy for these conditions to be made on the packaging in the absence of similar evidence to that required for conventional medicines under the Medicines Act 1968 and 1971. One small trial was claimed to suggest that the homeopathic use of Arnica to be no more effective than a placebo. In some quarters, the fact that homeopathic Arnica has been the subject of published clinical trials at all has drawn criticism grounded on the allegation that the basic premise of the high dilutions used in homeopathy would be inherently flawed. With respect to the range of homeopathic Arnica creams available on the market, these are generally formulated using the mother tincture rather than a dilution, and they therefore do in fact contain measurable quantities of the medicinally active substance.

This is another highly toxic essential oil, and is never used in Aromatherapy. It has great value in homoeopathy, where it is used both internally and externally in miniscule doses for shock, bruising and sprains. A jar of arnica cream is a 'must' for any plant-based medicine chest.