Arnica gel is a compound made from the Arnica plant and is sometimes used as a skin treatment to alleviate aches and pains, according to WebMD. Medicinal uses of Arnica are considered homeopathic and are not approved for clinical use in the United States, as of 2014.
The Arnica herb in its natural state is toxic to humans and can be fatal if ingested, according to WebMD. It is generally heavily diluted for homeopathic care, and Arnica gel has shown positive results in treating arthritis in the hands and knees. There is not enough research to consider it safe for children and pregnant or nursing women, as of 2014. Arnica has been associated with numerous side effects and may interact adversely with other drugs or supplements.