Calamus is an aromatic marsh herb used in some liqueurs, as well as scented beauty products and potpourri. In the United States, calamus is expressly prohibited for food use per the Code of Federal Regulations, but in other countries, it is allowed as long as the beta asarone is kept beneath a certain level.
Calamus is frequently cited as an ingredient in homemade reproductions of liqueurs including Campari, but as mentioned below the calamus most often available for sale is not safe for food use.
In addition to "sweet flag" and "calamus" other common names include beewort, bitter pepper root, calamus root, flag root, gladdon, myrtle flag, myrtle grass, myrtle root, myrtle sedge, pine root, rat root, sea sedge, sweet cane, sweet cinnamon, sweet grass, sweet myrtle, sweet root, sweet rush, and sweet sedge. [link]
Certain calamus varieties contain beta asarone, which was found to be "procarcinogenic" in animal studies [link]. Though certain varieties of calamus may be safer than others, calamus for sale is typically not labelled as to which type it is, and thus should be assumed to be dangerous to use in beverages.
From the International Programme on Chemical Safety [link]:
ß-asarone (cis-isomer of 2,4,5-trimethoxy-l-propenylbenzene) is a
constituent of oil of calamus, a flavouring agent derived from the
dried rhizome of Acorus calamus Linn. The ß-asarone content of
calamus oils varies with source of the plant. Indian Acorus calamus
from the Jammu area is tetraploid and yields an oil containing
approximately 75% ß-asarone; Acorus calamus from Kashmir is hexaploid
and yields an oil containing approximately 5% ß-asarone (Vashist &
Handa, 1964). The European variety of the plant is diploid and also
yields an oil with approximately 5% ß-asarone (Larry, 1973). Normally,
only the oil of the diploid variety is used for flavouring aromatic
alcoholic beverages (Usseglio-Tomasset, cited in Larry, 1973). The
roots and rhizomes of Acorus calamus have been used in the Ayurvedic
system of medicine for treating a variety of diseases such as epilepsy
add hysteria (Madan et al., 1960).
In the United States, calamus is included in “Substances Generally Prohibited From Direct Addition or Use as Human Food [link] by the FDA.”
(a) Calamus is the dried rhizome of Acorus calamus L. It has been used as a flavoring compound, especially as the oil or extract.
(b) Food containing any added calamus, oil of calamus, or extract of calamus is deemed to be adulterated in violation of the act based upon an order published in the Federal Register of May 9, 1968 (33 FR 6967).
Calamus is banned in Australia by name, as in the United States - meaning no amount is legal to use in food and beverages. Australia New Zealand Food Standards Code – Schedule 23 – Prohibited plants and fungi [link].
In Canada, "Oil, extract or root of calamus from Acorus calamus L." is forbidden in "all foods" as it is on the List of Contaminants and other Adulterating Substances in Foods [link].
European Union (as of 2002) [link]
Annex II of Directive 88/388/EEC on flavourings sets the following maximum levels for Beta-asarone in foodstuffs and beverages to which flavourings or other food ingredients with flavouring properties have been added: 0.1 mg/kg in foodstuffs and beverages, with the exception of 1 mg/kg in alcoholic beverages and seasonings used in snack foods. Beta-Asarone as such may not be added to foodstuffs (EEC, 1988).
"Opinion of the Scientific Committee on Food on the presence of -asarone in flavourings and other food ingredients with flavouring properties" [link to pdf]