Benefits
Prostate and urinary comfort in men
In a 6-month placebo-controlled trial in 620 men with prostate enlargement, symptom scores fell from 19.8 to 11.8 points on nettle against 19.2 to 17.7 on placebo; its sole author has had more than 20 other papers retracted. In a 12-month trial of a root extract in 246 men, scores fell 5.7 points against 4.7, a gap EU reviewers judged not clinically significant. A 60-man trial found an intermediate effect.
Urinary flow and leftover urine
In the 620-man trial, peak urine flow rose by 8.2 mL/s with nettle against 3.4 mL/s with placebo, and urine left in the bladder after voiding fell from 73 to 36 mL. In a 12-month trial of a root extract in 246 men, peak flow rose 3.0 against 2.9 mL/s and leftover urine fell by a similar amount in both groups, so the flow result has not been repeated.
Urinary symptoms with a saw palmetto and nettle root combination
In a 24-week double-blind trial in 257 older men with urinary symptoms from prostate enlargement, a capsule of 160 mg saw palmetto extract and 120 mg nettle root extract taken twice daily lowered symptom scores by about 6 points against about 4 on placebo. Because it is a combination, it cannot show the effect of nettle root alone.
Traditional EU use for urinary comfort in older men
The European Medicines Agency accepts nettle root preparations as traditional herbal medicines for lower urinary tract symptoms linked to prostate enlargement, to be used after a doctor has ruled out serious conditions, based on more than 30 years on the market. Its reviewers found the placebo-controlled trials too short, too small, run outside the EU or using poorly defined extracts to prove efficacy.
Mechanism of action
SHBG binding by root lignans
Several lignans from nettle root, and the gut metabolites enterodiol and enterolactone, bind human sex hormone binding globulin (SHBG) in test-tube assays. In a 9-week placebo-controlled trial in 50 men, blood SHBG fell on a root extract, while blood testosterone did not change in the 620-man trial.
Proposed prostate mechanisms
A 2007 review concluded that the mechanism is still unclear. It judged that SHBG, aromatase, epidermal growth factor and prostate steroid membrane receptors are likely involved, and that 5-alpha-reductase or androgen receptors are less likely to be. These conclusions rest on laboratory and animal work.
Anti-inflammatory activity
Nettle root extract and a polysaccharide fraction showed anti-inflammatory activity in laboratory studies. In the 60-man trial, the extract had a small effect on hs-CRP, a blood marker of inflammation.
UDA lectin
Urtica dioica agglutinin (UDA), a lectin isolated from the root, has immune-modulating effects in laboratory studies. Its role at supplement doses in people is unknown.
Clinical trials
Double-blind, placebo-controlled multicenter trial of PRO 160/120 (160 mg saw palmetto extract WS 1473 plus 120 mg nettle root extract WS 1031 per capsule), one capsule twice daily for 24 weeks, followed by 24 weeks of open treatment. (Lopatkin et al. 2005, World J Urol)
257 elderly men with lower urinary tract symptoms caused by benign prostatic hyperplasia.
Symptom scores (I-PSS) fell by about 6 points with the combination and about 4 with placebo after 24 weeks (P = 0.003, one-tailed), for both obstructive and irritative symptoms. The abstract does not report a significant difference in urine flow. Tolerability was similar to placebo. Because the product combines two herbs, the effect of nettle root alone cannot be isolated.
Double-blind, randomized, placebo-controlled, partial crossover trial of Urtica dioica for 6 months, with open treatment continued to 18 months. The abstract does not name the preparation; EU reviewers describe it as 120 mg of a root fluid extract three times daily but could not identify the exact preparation. (Safarinejad 2005, J Herb Pharmacother)
620 men with lower urinary tract symptoms from benign prostatic hyperplasia; 558 completed.
81% on nettle root reported improved urinary symptoms against 16% on placebo (P < 0.001). Symptom scores fell from 19.8 to 11.8 against 19.2 to 17.7 on placebo (P = 0.002), peak flow rose by 8.2 against 3.4 mL/s (P < 0.05), and leftover urine fell from 73 to 36 mL. PSA and testosterone did not change. No side effects were identified. The author called for further trials before concluding that nettle root is effective. This trial has not been retracted, but journals have retracted more than 20 of the same author's other papers, so independent confirmation matters.
Randomized, double-blind, placebo-controlled trial of 150 mg Urtidin root extract tablets three times daily (450 mg/day) or placebo for 12 weeks. The maker, Barij, made and supplied the tablets and placebo, and one author lists a Barij research center affiliation; a university funded the study. EU reviewers set the trial aside because the extract was not properly defined. (Akbar Karami et al. 2020, Pak J Biol Sci)
60 men with benign prostatic hyperplasia and symptom scores (IPSS) between 8 and 19, 30 per group; 56 completed.
Symptom scores (IPSS), the main outcome, fell more on the extract than on placebo (p < 0.001), an intermediate effect on the authors' scale; the text gives the comparison as a p value and effect size and plots the scores in a figure. hs-CRP (small effect), MDA and SOD also changed more than on placebo; other measured outcomes did not differ. No side effects were seen.
Randomized, double-blind, placebo-controlled multicenter trial of 459 mg of dry nettle root extract (Bazoton uno) for 12 months. Published in German; read from the English abstract and the EU assessment report, and funding is not stated in the abstract. (Schneider et al. 2004, Urologe A)
246 men with benign prostatic syndrome (lower urinary tract symptoms from an enlarged prostate).
Symptom scores (IPSS) fell from 18.7 to 13.0 on nettle root and from 18.5 to 13.8 on placebo (p = 0.0233). Peak urine flow rose 3.0 vs 2.9 mL/s and leftover urine fell from 35.5 to 20.0 mL vs 40.0 to 21.0 mL, neither different from placebo. Adverse events (29 vs 38) and urinary infections (3 vs 10 events) were fewer on nettle root. EU reviewers judged the gap in symptom score (31% vs 25% fall) not clinically significant.
Double-blind, placebo-controlled trial of ERU capsules (extractum radicis Urticae) for 9 weeks; EU reviewers give the dose as 600 mg of extract daily. Published in German; read from the English abstract and the EU assessment report, and funding is not stated in the abstract. (Vontobel et al. 1985, Urologe A)
50 men with stage I to II benign prostatic hyperplasia who had been referred to assess the need for surgery.
Urinary complaints improved in both groups, and EU reviewers found no notable difference between groups in symptoms. Blood SHBG, a hormone binding protein, fell on the extract (p = 0.0005) and voided volume rose by 44%, a significant change. The abstract reports a significant gain in peak urine flow, but EU reviewers give p = 0.062 for that measure and say the study cannot be fully assessed because complete data were not published. Leftover urine rose in both groups. Four participants reported digestive complaints.
Randomized, double-blind, placebo-controlled trial of 150 mg Urtidin tablets four times daily for 1 month, added to loratadine 10 mg daily and saline nasal rinses in both groups. The title calls it a root extract, and a separate trial describes Urtidin as a dried root extract. Barij Essence Pharmaceutical made both the tablets and the placebo; a university funded the study. (Bakhshaee et al. 2017, Iran J Pharm Res)
74 people with allergic rhinitis and a positive skin prick test whose symptoms had responded little to antihistamines and nasal steroids; 40 completed.
Sino-nasal symptom scores (SNOT-22) improved in both groups, from 44.4 to 23.1 on nettle root and from 43.5 to 27.6 on placebo, with no significant difference between groups (p = 0.25). Nasal smear eosinophils fell within the nettle group but did not differ significantly from placebo, and IgE, IL-4 and IL-5 did not change. Interferon gamma rose slightly on nettle root and fell on placebo (between groups p = 0.017). No serious adverse effects were reported.