The family Cannabaceae comprises Hops (Humulus) and hemp (Cannabis). The Humulus genus is confined to the temperate and subtropical regions of the northern hemisphere. All are Eurasian natives. There are only 3 species: European hop (H. lupulus), found throughout Europe; Japanese hop (H. scandens syn. H. japonicus), found in Japan and throughout most of China; and Yunnan hop (H. yunnanensis), native to Yunnan province. (1)

Native to Europe and western Asia, the Hop plant is now cultivated in North and South America, Africa, Asia and Australia. Both H. lupulus and H. scandens have escaped cultivation and are found throughout the eastern United States and Canada, and as far west as Manitoba. (1)

The plant is a perennial climber growing to 6 m at a medium rate. Leaves are opposite, deeply divided into 5-7 palmate lobes, with serrate edges and a rough upper surface and pubescent underside, on a long petiole. Hop is dioecious, with separate male and female plants. Male flowers are yellow-green, arranged on 15-25 cm-long, narrowly spreading panicles. Female flowers are catkin-like drooping spikes 5 mm in diameter. The plant is entirely wind-pollinated. (1)

“Hops” is the common term for either the dried flower heads as a whole or the extract, with a bitter taste and aromatic odour, from the dried pine cone-like fruit of the plant. Hops contribute flavour and aroma and act as a preservative in brewed alcoholic beverages, and are used medicinally mainly to treat sleep disturbances. (2) The extract may be in solid, liquid or oil form.

Hop grows in hedgerows, woodlands and sunny waste ground, as well as in cultivated fields, where it is trained on high trellises. The manufacture of beer utilises 98% of the world's production of Hops. Before the days of pasteurisation, brewers used Hops for their antibiotic properties as well as their flavour. In some countries the young shoots, heads, leaves and roots of the plant are eaten as vegetables. (The leaves contain rutin and if eaten raw should be very fresh.) A tea is made from the leaves and cones. Extracts from the plant, including the oil, are used as flavouring in non-alcoholic beverages, frozen dairy desserts, candy, baked goods, puddings and tobacco. The seeds contain gamma-linolenic acid, an essential fatty acid that is said to have many important functions in the human body and is rarely found in plant sources.

Hops are said to have various biological activities. (The antimicrobial activities are due to the bitter acids, especially lupulone and humulone, which give Hops their aroma and beer its bitter flavour.) Hops are widely used as a folk remedy to treat a range of complaints, including boils, cancer, cough, leprosy, tuberculosis, diarrhoea (including acute dysentery), toothache, jaundice, rheumatism, and worms. Hop flowers are popular as an infusion and can also be used to stuff pillows, so that the weight of the head will release the volatile oils.

See under Environment. Also, a brown dye is obtained from the leaves and flower heads. An essential oil is used in perfumery. Extracts of the plant are used in Europe in skin creams and lotions for their alleged skin-softening properties.

Recently, counterculture entrepreneurs have apparently succeeded in grafting Hops tops onto marihuana plants and getting a "heady Hop". Conversely, they might have succeeded in getting a perennial marihuana by grafting the annual herb onto the perennial Hop.

Allergen Description

No allergens from this plant have yet been characterised.

Potential Cross Reactivity

An extensive cross-reactivity among the different individual members of the family could be expected. (3) Members of this family include Hemp (Cannabis sativa) and Japanese Hop (Humulus scandens).

In a Korean study of 471 patients sensitised to Japanese hop (Humulus scandens), Mugwort, or Ragweed pollens on skin-prick testing, a relationship of Japanese hop with the foods Celery, Hop, and Sunflower pollen on skin-prick testing was demonstrated. The authors suggest that further investigations will be needed to evaluate the possibility of cross-reacting components. (45)

Clinical Experience

IgE-mediated reactions

Hops may uncommonly induce symptoms of food allergy in non-occupationally sensitised individuals. Occupational allergy may commonly be associated with exposure to Hops. (6) Hops are an uncommon cause of occupational asthma and anaphylaxis. (7) Hops have usually been picked by hand. (However, more recently, picking machines have been introduced in some countries.) Skin contact with the plant causes dermatitis in susceptible people.

Systemic and contact urticaria have been documented. Contact urticaria from dried Hops occurred in a patient who had experienced urticaria and angioedema from Peanut, Chestnut and Banana. (7) A patient who presented 4 times with systemic urticaria associated with arthralgia and fever has been reported; investigation confirmed allergy to Hops. The authors suggest that in some instances the adverse effects of Hops are not due to allergy but to another mechanism that is unclear - the constituent lupuline may play a role. (8)

Hops dermatitis has long been recognised. It is a mechanical dermatitis attributed to the rough hairs on the stem and secretions of the yellow glandular hairs. (7, 9) Hop pickers can also develop a dermatitis from the leaves, as well as conjunctivitis and tenosynovitis, which is thought to be irritant rather than allergic, and due to the myrcene oxidation products humulone and lupulone. (1) Of 156 Ukrainian Hop workers, 15% were found to have Hop-related skin diseases. (10) Among 14 Polish farmers complaining of work-related skin symptoms, these were caused most often by Hops (11%), followed by grain (5.6%), hay (5.5%) and straw (4.1%). Five farmers (6.8%) complained of hand dermatitis, 4 (5.5%) of airborne dermatitis, and 8 (11.0%) of pruritus. On skin prick tests, 5.5% of farmers reacted to grain dust, 5.5% to straw dust, 11% to hay dust, and 8.2% to Hops. (11) Not only the hands and face but the legs may be involved due to Hop picking. Dislodged hairs from the plant can irritate the eyes.

In 73 eastern-Polish Hop farmers, 8 (11%) complained of skin symptoms provoked by contact with Hop. Four of them suffered from airborne dermatitis, 2 had hand eczema, and the remaining 2 complained of intense pruritus of uncovered skin when working with Hop. Two of the 8 farmers had had positive prick tests with Hop extracts. (12 , 13)

The onset of occupational airborne dermatitis and hand dermatitis to Hops in a 57-years-old female farmer has been documented. The disease appeared at the age of 46, 30 years into working with Hops without any health problems. The patient had erythema of the face, neck and upper chest, oedema of the eyelids, and conjunctivitis, as well as acute dermatitis of the hands. Both fresh and dried Hops precipitated the symptoms. Onset occurred after half an hour of working and persisted over 1-2 days. Despite discontinuing work, the patient experienced several relapses of her dermatitis, and this was attributed to her use of a beauty cream and a herbal sedative, which both contained Hops extract. Sleeping in a bed with her husband, who worked with Hops, provoked relapses of the patient's dermatitis. (The husband sometimes felt too tired to wash thoroughly after working on the plantation.) (13)

Anaphylaxis following exposure to Hops has been reported, and should be considered in individuals with “idiopathic” anaphylaxis. (14)

Occupational allergy to Hops has been reported. A chemist developed urticaria, rhinitis, conjunctivitis and asthma after 6 months’ work as a Hop selector for a brewery, (6) and a Hop farmer had occupational respiratory disorder. (15) A laboratory worker was also described who developed conjunctivitis, rhinitis, bronchitis and dermatitis to Hops. (16)

Other reactions

Hop leaves contain lupin and rutin. Sometimes Hops are treated with sulphur dioxide to improve the colour and prevent change of active substances.

Hops contain a phytooestrogen. This substance can also be detected in beer, but the levels are low and should not cause concern. (17) In animal studies, approximately 500-fold greater levels were required for oestrogenic effects than can be found in any beer. (18) The presence of 8-prenylnaringenin in Hops may provide an explanation for the accounts of menstrual disturbances in female Hops workers.

Of 23 Hops farmers exposed to organic dust from Hops, 5 (21.7%) reported work-related symptoms, including dry cough and dyspnoea. Eight farmers (34.8%) reported symptoms of chronic bronchitis. Spirometric values were within normal ranges. Precipitin tests and tests for inhibition of leukocyte migration to antigens of environmental microbes, mainly to the antigen of the Gram-negative bacterium Pantoea agglomerans, were positive. (19)

Last reviewed: June 2022.