Rhein

Pharmacology & Research

Rhein is an anthraquinone aglycone — a sibling of emodin — found in rhubarb, yellow dock, senna and Cassia ^{13,14Reference 13Qian X et al. · 2024ReviewRumex crispus L.: a comprehensive review on botany, traditional uses, phytochemistry, pharmacology and safetyView study →Reference 14Vasas A et al. · 2015ReviewThe genus Rumex: review of traditional uses, phytochemistry and pharmacologyView study →}, usually bound in the intact plant as glycosides or sennoside dianthrones. What sets it apart from the other plant anthraquinones is a drug connection: rhein is the active metabolite of diacerein (diacetylrhein), a licensed slow-acting osteoarthritis drug, so it uniquely carries human clinical data by proxy ^{1Reference 1Nicolas P et al. · 1998Clinical pharmacokinetics of diacereinView study →}. Two caveats frame that data throughout the page. First, every human result is for diacerein, not isolated plant-derived rhein — and even for diacerein the osteoarthritis benefit is small and disputed ^{4Reference 4Fidelix TS et al. · 2014Systematic reviewDiacerein for osteoarthritisView study →}. Second, isolated rhein has poor aqueous solubility and low oral absorption; the favourable human pharmacokinetics belong to the diacerein delivery form ^{8Reference 82024ReviewRhein: an updated review concerning its biological activity, pharmacokinetics, structure optimisation and future pharmaceutical applicationsView study →}.

1. Osteoarthritis (via diacerein)

Diacerein — the diacetyl prodrug for which rhein is the systemically active moiety — is a licensed symptomatic slow-acting drug for osteoarthritis. Earlier meta-analyses found a small, consistent benefit on pain and function ^{3,5Reference 3Rintelen B et al. · 2006Meta-analysisA meta-analysis of controlled clinical studies with diacerein in the treatment of osteoarthritisView study →Reference 5Bartels EM et al. · 2010Meta-analysisSymptomatic efficacy and safety of diacerein in the treatment of osteoarthritis: a meta-analysis of randomised placebo-controlled trialsView study →}, but the 2014 Cochrane review concluded the symptomatic benefit is minimal-to-none versus placebo, with a possible small structural (joint-space) signal in hip OA and diarrhea as the dominant harm ^{4Reference 4Fidelix TS et al. · 2014Systematic reviewDiacerein for osteoarthritisView study →}.

Gap: all human data are for diacerein, not isolated plant-derived rhein; the effect size is small and disputed, and there are no trials of rhein itself ^{4Reference 4Fidelix TS et al. · 2014Systematic reviewDiacerein for osteoarthritisView study →}.

2. Anti-inflammatory (IL-1β)

The mechanism behind the OA rationale. Rhein downregulates IL-1β signalling — fewer surface IL-1 receptors, reduced IL-1 binding, increased IL-1 receptor antagonist, and suppressed downstream metalloproteinase and nitric-oxide production in cartilage models ^{6Reference 6Martel-Pelletier J · 2010Effects of diacerein at the molecular level in the osteoarthritis disease processView study →}.

Gap: the mechanism is well characterised in joint tissue but does not translate to a large clinical anti-inflammatory effect, and there are no standalone anti-inflammatory trials of rhein ^{6Reference 6Martel-Pelletier J · 2010Effects of diacerein at the molecular level in the osteoarthritis disease processView study →}.

3. Diabetic nephropathy / renal

Comparatively strong preclinically. A systematic review and meta-analysis of animal models found rhein associated with improved diabetic-nephropathy outcomes ^{10Reference 10Hu Y et al. · 2019Meta-analysisA significant association between rhein and diabetic nephropathy in animals: a systematic review and meta-analysisView study →}, and rhein reverses Klotho repression via promoter demethylation, protecting kidney and bone in chronic-kidney-disease mice ^{11Reference 11Zhang Q et al. · 2017AnimalRhein reverses Klotho repression via promoter demethylation and protects against kidney and bone injuries in mice with chronic kidney diseaseView study →}. The single human RCT (diacerein 50 mg twice daily for 90 days in type-2-diabetic CKD) did not improve the albumin/creatinine ratio or GFR but slowed metabolic-control deterioration and lowered night-time blood pressure ^{2Reference 2Piovesan AP et al. · 2017RCTEffect of diacerein on renal function and inflammatory cytokines in type-2 diabetes and chronic kidney disease: a randomised controlled trialView study →}.

Gap: the human evidence is a single small trial that was negative on its primary endpoints and used the prodrug — efficacy in humans is unproven ^{2Reference 2Piovesan AP et al. · 2017RCTEffect of diacerein on renal function and inflammatory cytokines in type-2 diabetes and chronic kidney disease: a randomised controlled trialView study →}.

4. Anticancer

Rhein induces cell-cycle arrest and mitochondrial/ROS-dependent apoptosis across many tumour lines via caspase activation and NF-κB/p53 modulation ^{7Reference 7Henamayee S et al. · 2020ReviewTherapeutic emergence of rhein as a potential anticancer drug: a review of its molecular targets and anticancer propertiesView study →}.

Gap: entirely cell and animal, with poor solubility/bioavailability discounting the in-vitro potency and no human oncology data ^{7Reference 7Henamayee S et al. · 2020ReviewTherapeutic emergence of rhein as a potential anticancer drug: a review of its molecular targets and anticancer propertiesView study →}.

5. Laxative (class)

Rhein alters intestinal ion transport and colonic motility ^{9Reference 9Tsai TH et al. · 2004AnimalEffect of ethanol extracts of three Chinese medicinal plants with laxative properties on ion transport of the rat intestinal epitheliaView study →}, but the purgative action of rhubarb and senna is driven by the anthraquinone glycosides converted by colonic flora to rhein-anthrone, not by the free aglycone absorbed upstream ^{12Reference 122021Pharmacokinetics of anthraquinones from medicinal plantsView study →}.

Gap: free rhein is not a meaningful stand-alone laxative — the relevance is class-contextual ^{12Reference 122021Pharmacokinetics of anthraquinones from medicinal plantsView study →}.

Mechanisms

Target / pathway	Effect	Relevant to
IL-1β / IL-1 receptor signalling	↓ surface IL-1 receptors, ↑ IL-1Ra; ↓ IL-1-driven MMP and NO in cartilage	osteoarthritis, anti-inflammatory
NF-κB	↓ inflammatory transcription; ROS-linked NF-κB/p53 modulation in tumour cells	anti-inflammatory, anticancer
Mitochondrial / ROS–caspase apoptosis	caspase-3/-9, PARP cleavage	anticancer (preclinical)
Klotho promoter demethylation	reverses Klotho repression → renoprotective, anti-fibrotic	diabetic nephropathy
Intestinal ion transport / motility	altered water-electrolyte transport, prokinetic (class)	laxative (class)

Pharmacokinetics

Load-bearing, with a key distinction: rhein is relatively well absorbed for an anthraquinone, but the human data describe rhein delivered as diacerein, not free plant rhein. Diacerein is essentially completely deacetylated to rhein before and at first pass, so rhein is the only species reaching the circulation in meaningful amounts ^{1Reference 1Nicolas P et al. · 1998Clinical pharmacokinetics of diacereinView study →}; absolute bioavailability is roughly 35–56% depending on formulation (food delays but slightly increases absorption), plasma protein binding is ~99% and non-saturable, and rhein is glucuronidated (~60%) and sulfated (~20%) with a steady-state half-life of ~7–8 hours ^{1Reference 1Nicolas P et al. · 1998Clinical pharmacokinetics of diacereinView study →}. The free-aglycone caveat is decisive: isolated rhein itself has poor aqueous solubility and low oral absorption, so much of the favourable human pharmacokinetics is attributable to the diacerein form ^{8Reference 82024ReviewRhein: an updated review concerning its biological activity, pharmacokinetics, structure optimisation and future pharmaceutical applicationsView study →}.

Clinical trials

There are no clinical trials of isolated, plant-derived rhein — the human evidence is entirely diacerein: positive-but-modest OA meta-analyses ^{3,5Reference 3Rintelen B et al. · 2006Meta-analysisA meta-analysis of controlled clinical studies with diacerein in the treatment of osteoarthritisView study →Reference 5Bartels EM et al. · 2010Meta-analysisSymptomatic efficacy and safety of diacerein in the treatment of osteoarthritis: a meta-analysis of randomised placebo-controlled trialsView study →}, the skeptical 2014 Cochrane review ^{4Reference 4Fidelix TS et al. · 2014Systematic reviewDiacerein for osteoarthritisView study →}, and a diabetic-CKD RCT that was negative on its primary endpoints ^{2Reference 2Piovesan AP et al. · 2017RCTEffect of diacerein on renal function and inflammatory cytokines in type-2 diabetes and chronic kidney disease: a randomised controlled trialView study →}. Potency of the isolated molecule or of the raw herb should not be inferred from these prodrug trials.

Last checked: July 2026.

Toxicity & Safety

Rhein’s safety profile is dominated by its behaviour as diacerein, which is why this page is flagged moderate. Diarrhea is the dominant adverse effect — frequent, dose-related and sometimes severe, consistently the top harm across the OA meta-analyses and the Cochrane review ^{4Reference 4Fidelix TS et al. · 2014Systematic reviewDiacerein for osteoarthritisView study →}. Diacerein also carries a hepatotoxicity signal (raised transaminases and cases of acute liver injury). Following a French-initiated referral over gastrointestinal and liver toxicity, the EMA in 2014 kept diacerein’s benefit–risk positive but restricted: it is not recommended in patients ≥65 years, should be started at half dose (50 mg/day) and stopped if diarrhea occurs, and is contraindicated in current or prior liver disease with liver monitoring advised (it is not FDA-approved in the US). As an anthraquinone it also sits within the stimulant-laxative class concerns (electrolyte loss, dependence with chronic use, pseudomelanosis coli), echoing the hydroxyanthracene-derivative context noted on the emodin page. The high but non-saturable protein binding makes displacement interactions unlikely ^{1Reference 1Nicolas P et al. · 1998Clinical pharmacokinetics of diacereinView study →}; the practical interaction concerns are additive diarrhea (other laxatives) and co-administered hepatotoxic drugs.

Pregnancy & lactation

Avoid. Anthraquinone metabolites including rhein are excreted into breast milk ^{12Reference 122021Pharmacokinetics of anthraquinones from medicinal plantsView study →}, stimulant anthraquinones are generally advised against in pregnancy and lactation, and diacerein has inadequate human pregnancy safety data — not recommended in either.

Dosage

There is no established or safe dose for isolated plant-derived rhein, and nothing here is a recommendation. For reference, the licensed prodrug diacerein is dosed at 50 mg twice daily for osteoarthritis, but under the EMA restriction it should be initiated at 50 mg/day, discontinued if diarrhea occurs, and avoided in patients ≥65 years and in liver disease ^{4Reference 4Fidelix TS et al. · 2014Systematic reviewDiacerein for osteoarthritisView study →}. Each 50 mg of diacerein delivers roughly one molar equivalent of rhein — but those figures describe an approved drug under regulatory restriction, not a supplement dose.

References