Heavy metal contamination in herbal medicines: a systematic review and meta-analysis

Herbal medicine products (HMPs) are widely consumed worldwide, yet contamination with toxicants such as heavy metals poses potential environmental health and consumer safety concerns. This study systematically reviewed studies measuring contamination levels of lead (Pb), cadmium (Cd), mercury (Hg), or arsenic (As) in crude herbal materials and formulated HMPs. PubMed, Scopus, Web of Science, and CINAHL were searched from inception to June 2026. Random-effects meta-analyses estimated metal-specific pooled mean concentrations; moderator analyses and meta-regression explored heterogeneity. Seventy studies from 27 countries or territories were included. Forty-eight, 45, 25, and 35 studies contributed to the Pb, Cd, Hg, and As meta-analyses, respectively. Pooled concentrations in HMPs were 1.930 mg/kg (95 % CI: 0.993-2.860) for Pb, 0.206 mg/kg (95 % CI: 0.142-0.270) for Cd, 0.122 mg/kg (95 % CI: 0.016-0.228) for Hg, and 0.501 mg/kg (95 % CI: 0.267-0.736) for As; heterogeneity was considerable for all metals. Pb concentrations differed by national income group (p=0.007), and Cd concentrations differed by product classification (p=0.002), with higher Cd concentrations in crude materials than formulated products. Higher gross national income per capita was associated with lower Pb and As concentrations. Reported exceedance of study-specific comparators was common but varied substantially because regulatory limits, product categories, and denominators differed across studies. Heavy metal contamination of HMPs varies between settings and products, which underscores the need for strengthened quality surveillance, improved monitoring, and harmonized regulatory standards.