Sub-acute Toxicity of Licorice-Sargassum Extract in Sprague-Dawley Rats: Biochemical, Histopathological, and Pharmacokinetic Studies

Traditional Chinese Medicine (TCM) has guided herbal therapy for millennia, including the “eighteen antagonistic medicaments”—a critical set of rules warning against combining specific herbs. One of the most well-known prohibitions? Never mix licorice (Glycyrrhiza glabra), a sweet, anti-inflammatory herb, with sargassum, a brown seaweed used to clear phlegm. But here’s a paradox: a classic TCM recipe called Hai-Zao-Yu-Hu-Tang, used to treat thyroid tumors and breast hyperplasia, relies on both. Is this combination safe? A 2022 study by researchers from Nanjing University of Chinese Medicine sought to answer that question—with worrying results for anyone using these herbs together.

Led by Xinyue Jing (Nanjing University of Chinese Medicine) and Yunru Peng (Jiangsu Key Laboratory for High Technology Research of TCM Formulae), the team tested the sub-acute toxicity (harm from repeated use over weeks) of licorice-sargassum extract in Sprague-Dawley rats. They focused on three key areas: biochemical markers (blood tests for organ health), histopathology (tissue damage), and pharmacokinetics (how the body absorbs and processes the herbs).

How the Study Worked

The researchers divided 7 groups of rats to receive:

Normal saline (control)
Low/high doses of sargassum alone (2.66 g/kg or 5.33 g/kg)
Low/high doses of licorice alone (2.42 g/kg or 4.83 g/kg)
Low/high doses of combined licorice-sargassum extract (4.37 g/kg or 8.75 g/kg)

Rats got doses twice daily for 4 weeks. Afterward, the team:

Measured organ health: Calculated “organ coefficients” (organ weight relative to body weight) to check for swelling or damage.
Ran blood tests: Checked for markers of heart (creatine kinase), liver (alanine transaminase), and kidney (blood urea nitrogen) stress.
Examined tissues: Used hematoxylin-eosin (H&E) staining to see cell damage and CD68 immunohistochemistry to measure inflammation (CD68 is a marker for macrophages, immune cells that signal inflammation).
Tracked herb compounds: Used ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-TQ/MS) to measure six key licorice compounds in plasma—including glycyrrhizic acid (GL) and its metabolite glycyrrhetinic acid (GA), which are linked to licorice’s effects.

What the Study Found

The results painted a clear picture: high-dose licorice-sargassum extract (HLS) was toxic to the heart, liver, and kidneys.

1. Biochemical Markers: Signs of Organ Stress

Rats in the HLS group had:

Higher heart damage markers: Elevated creatine kinase, hydroxybutyrate dehydrogenase, and lactate dehydrogenase (LDH)—all signs of heart muscle stress.
Higher liver damage markers: Increased alanine transaminase (ALT) and aspartate aminotransferase (AST)—enzymes released when liver cells are damaged.
Higher kidney damage markers: Raised blood urea nitrogen (BUN) and creatinine—waste products that build up when kidneys aren’t working well.
Metabolic disruption: Higher triglycerides (fat) and glucose—suggesting the combination messes with sugar and lipid metabolism.

Even low-dose combined extract (LLS) caused milder but still significant increases in these markers.

2. Histopathology: Tissue Damage and Inflammation

The HLS group had the worst tissue damage:

Heart: Severe inflammation (neutrophil granulocyte infiltration) and destroyed heart cells.
Liver: Bile duct inflammation and liver cell degeneration—signs of early cirrhosis.
Kidney: Pyelonephritis (kidney infection), thickened tubules, and inflammatory cell clusters around kidney structures.

CD68 staining confirmed more macrophages in HLS organs—meaning stronger inflammation driving the damage.

3. Pharmacokinetics: Sargassum Boosts Toxic GA Levels

Licorice’s main active compound is GL, which the body converts to GA in the gut. GA is problematic because its chemical structure is similar to corticosteroids—too much can cause “false aldosteronism” (a hormone imbalance leading to high blood pressure, low potassium, and organ damage).

The study found:

Licorice-sargassum extract raised GA levels: Rats given the combined extract had significantly higher plasma GA (area under the curve, or AUC, and peak concentration, Cmax) than those given licorice alone—even after multiple doses.
GL absorption changed: A single dose of combined extract increased GL levels, but repeated doses slightly lowered GL (likely because GA buildup blocked further GL absorption).

This matches earlier research where laminaria (a seaweed similar to sargassum) also boosted GA levels when mixed with licorice.

Why This Matters for TCM and Herbal Safety

The study’s key takeaway: combining licorice and sargassum may be toxic because sargassum increases absorption of GA—the compound responsible for licorice’s side effects.

But here’s the clinical context: Hai-Zao-Yu-Hu-Tang uses specific proportions of licorice and sargassum. Could lower doses or different ratios be safe? The study doesn’t answer that—but it highlights a critical gap: we need more research to understand how TCM recipes balance therapeutic effects and toxicity.

For now, the results are a warning: if you’re using TCM herbs that combine licorice and sargassum, talk to a licensed practitioner about safety. Repeated use of high doses could harm your heart, liver, or kidneys.

The Bottom Line

The 2022 study from Nanjing University of Chinese Medicine adds important data to the debate over licorice-sargassum safety. While classic TCM recipes have value, modern science is essential to confirm their safety—especially for combinations the “eighteen antagonistic medicaments” warned against.

The study was published in the Chinese Medical Journal in 2022. Authors: Xinyue Jing, Xian Meng, Zhenhui Wu, Yongfang Ding, Yunru Peng, Mingqing Shen, Qing Wang. doi.org/10.1097/CM9.0000000000001716

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