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By Dónal P. O’Mathúna, PhD
The stinging nettle, that distinctive dark green, hairy plant found in wastelands the world over, has brought many children to tears and adults to swearing. In fact, the term urticaria is derived from nettle’s official name, Urtica dioica, and describes the type of rash produced by nettles. Paradoxically, in the United States, nettle has developed a reputation for the relief of hay fever symptoms (allergic rhinitis). Anything mild and effective that relieves the irritating symptoms of hay fever would be widely welcomed.
A medicinal and culinary plant, stinging nettle has been used around the world to treat asthma, allergies, coughs, rheumatism, and the symptoms of benign prostatic hyperplasia (BPH). It has been given as a diuretic and an antispasmodic.1 Both nettle root and the above-ground parts are used, although their constituents are somewhat different. During the 19th century, the principle of similarity (think bushy prongs) led to widespread use of nettle juice to stimulate hair growth and to the traditional recommendation that paralyzed limbs be whipped with nettles.2 Nettles remain a staple vegetable in certain countries and are commonly incorporated into soups. Young nettle shoots are nutritious and contain as much carotene and vitamin C as spinach and other greens.1
The nettle hair is a capillary with a bladder at the base and a tiny bulb at the tip.3 When the bulb is broken off by contact, a fine needle-like structure is exposed which penetrates the skin and injects a fluid. This fluid’s composition is not completely established, but it contains acetylcholine, histamine, and serotonin (5-hydroxytryptamine).4 Nettle stings are more severe and long-lasting than those induced by synthetic mixtures of these compounds, leading to suspicion that the sting also contains a potent histamine-releasing compound.
Nettles contain a wide variety of other compounds, including formic acid, b-sitosterol and other sterols, coumarin (scopoletin), flavonoids, carbohydrates, amino acids, and a relatively high amount (2.7%) of chlorophyll.5 The root also contains a lectin (a large sugar-protein compound) called Urtica Dioica Agglutinin (UDA) which has been shown to inhibit prostaglandin and leukotriene synthesis.6 UDA and b-sitosterol are believed to underlie nettle’s use for rheumatism and BPH. Other active ingredients in nettle have not been identified, and a mechanism of action for nettle in hay fever is not known.
In Germany, several observational studies and two randomized, double-blind studies support the use of nettle root as a diuretic and for symptomatic relief of mild BPH.7 But searches of MEDLINE, International Pharmaceutical Abstracts, Toxline, and The Cochrane Library (using nettle and urtica) produced only one clinical trial using nettle to treat hay fever.8
Volunteers were sought in Oregon during hay fever season (May through early July). Subjects were required to have at least moderately severe symptoms in two of the following three categories: rhinorrhea, sinus congestion, and excessive lacrimation. The study accepted 98 subjects who were randomly assigned to receive capsules containing either placebo or 300 mg freeze-dried Urtica dioica in a preparation containing primarily leaves and stems. According to a conversation with P. Mittman (June 2000), a random number list was used for group assignment, and both patients and researcher were blinded. Subjects were asked if they thought they received placebo, but their answers were not published.
Subjects were instructed to take two capsules upon onset of hay fever symptoms; within one hour subjects were told to evaluate and record their responses as either dramatic improvement, moderate improvement, no change, or worse. Subjects answered other general questions at the end of the trial, the duration of which was not reported.
Of the 98 participants, 69 finished the study, 31 taking nettle and 38 on placebo. Twenty were lost to follow-up, seven took fewer than five doses and felt unable to evaluate their product, and two withdrew because of side effects. The 69 subjects took between five and 34 doses, or an average of 17.8 doses, during the trial period. The average daily dose was 2.8, with a range of 1 to 7. It was not stated if a "dose" was 1 or 2 capsules.
The percentage of people in the nettle group who reported at least one instance of each type of symptom change were: dramatic improvement (32%), moderate improvement (84%), no change (74%), or worse (32%). For those taking placebo, the results were: dramatic improvement (16%), moderate improvement (71%), no change (84%), or worse (36%).
Participants were asked to make an overall assessment of their product. Of those taking nettle, 13 (42%) rated it as ineffective and 18 (58%) as moderately or highly effective. For the placebo, 24 (63%) rated it as ineffective and 14 (37%) as moderately or highly effective. When asked to compare the product with previous hay fever medicines used, of those taking nettle, 16 (52%) rated it as less effective and 15 (48%) as the same or more effective. For the placebo, 30 (79%) rated it as less effective and 8 (21%) as the same or more effective. Statistical analysis of the results was not published, but according to a conversation with P. Mittman (June 2000), chi-squared tests showed that the differences between nettle and placebo were statistically significant (P < 0.05).
In the above study, seven people taking nettle and five taking placebo reported mild gastric discomfort when taking the capsules on an empty stomach.8 Two people withdrew because the nettle intensified their allergy symptoms. A study of nettle extract for BPH reported side effects in less than 1% of 4,087 patients.7 The most common effects were gastrointestinal complaints and skin allergies. Nettle has a folk use as an abortifacient, and uterine contractions have been observed in animal studies.9 For this reason, women who are or who want to be pregnant should not use nettle products.
The diuretic effect of nettle is well established; thus, it may interfere with other diuretics and heart failure medications. The presence of coumarin and the many other compounds suggests an interaction with anticoagulant therapy is likely. Animal studies have documented CNS-depressant activity for nettle, which may potentiate other drugs with these effects.9 Anecdotal reports of nettle have shown both hypoglycemic and hyperglycemic activity, making its use by diabetic patients unwise.
Nettle is available in capsules, as a dried leaf and root extract, or as a tincture. Nettle products are recommended for use as needed, with no limit suggested. A tea can be made from the dried herb, with 2-4 g used three times daily for medicinal effects. Teas, however, are notorious for their imprecise drug-delivery dosages.
In Germany, an alcohol extract of nettle root is approved as a drug for use as a diuretic, especially for the symptomatic alleviation of BPH.
Stinging nettle has a long folk medicine reputation. It appears to be well tolerated, although allergic skin reactions can occur. Its use for the relief of hay fever symptoms is based on anecdotal evidence and one controlled study. Although well-designed, this study measured no objective outcomes, the dropout rate was 30%, and the study’s report omitted many important details. The placebo group had similar results, even reporting similar numbers and types of side effects. However, 32% of the placebo group had mild hay fever symptoms initially, whereas only 10% of the nettle group had mild symptoms. This might have influenced the results, but without adequate power and other statistical analysis, the significance of the results is uncertain.
Can nettles compensate for all the tears they cause in the wild by drying up the secretions of hay fever sufferers? Probably not. But nettle products may offer some symptomatic relief for some people with hay fever. A one-week trial like that in the study reported here may be appropriate, especially in otherwise healthy patients who are not and who do not want to be pregnant. However, the evidence suggests a trial with a placebo might be just as effective. Nettle seems to be well tolerated, although patients should watch for allergic reactions. For patients taking medications for other conditions, especially cardiac conditions, the potential for drug interactions outweighs the uncertain chance of benefit.
Dr. O’Mathúna is Professor of Bioethics and Chemistry at Mount Carmel College of Nursing, Columbus, OH.
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