Juniper is an evergreen shrub or tree native to Europe and naturalized in North America. Although juniper is most often found as a spreading, low-lying shrub, there are tall varieties that may reach as much as ten meters in height. The stems and branches of the plant are covered with red bark, which is often coated with sticky sap. The leaves are small and needle-like, 12-20 millimeters long. The plant flowers from April through June, with male and female flowers on separate plants. Male flowers grow in yellow catkins consisting of several stamens in three-segmented whorls. The green, ovoid female flowers, which consist of three carpels, eventually form the juniper berries. The berries, which are used medicinally and as the principle flavoring agent in gin, are bluish-white when ripe, and usually contain two or three angular seeds. Because it may take as long as three years for juniper berries to mature, ripe and green berries are commonly found on the same plant.
The primary active constituent in juniper berries is their volatile oil, which is composed primarily of monoterpene hydrocarbon compounds. Juniper berries also contain diterpenes, tannins, and flavonoids.
The daily dosage approved by Commission E is 2 to 10 grams of dried fruit or 20 to 100 mg of the essential oil. Duration of usage should be limited to six weeks, because long-term administration may cause kidney damage. Juniper is not recommended for use during pregnancy or for people with inflammation of the kidneys. Juniper contains varying amounts of vitamin K, which may interfere with the activity of anticoagulant drugs used to treat hypertension.