If you've spent any time reading about Pau d'Arco, you've run into the word lapachol. It shows up in old cancer research, in toxicity studies, in forum arguments about whether the tea is safe to drink. It's also the most-cited compound in Pau d'Arco's scientific history, so it's worth going through the studies instead of repeating what everyone else says about it.
Here's what those studies say, where they came from, and what they do and don't tell us about a cup of brewed tea.
What is Lapachol?
Lapachol is a naphthoquinone, a class of compounds built around a two-ring chemical structure found throughout the plant kingdom (vitamin K is probably the most familiar relative). It was first isolated in 1882 by the Italian chemist Emanuele Paterno, working with bark from Tabebuia avellanedae, the tree now classified as Handroanthus impetiginosus and the same species Pau d'Arco bark comes from. Its exact structure wasn't worked out until 1896, when the chemist Samuel C. Hooker identified it as 2-hydroxy-3-(3-methylbut-2-enyl)-1,4-naphthoquinone.
Lapachol isn't unique to Handroanthus. It's turned up in bark and wood across several unrelated plant families since (Girard et al., Journal of Natural Products, 1988). But Pau d'Arco remains its best-known natural source.
How much ends up in a cup of tea
The wood, bark, and roots of the tree contain roughly 1.5–2% lapachol by weight. That sounds like a lot until you brew it. Naphthoquinones like lapachol are poorly water-soluble. They need sustained heat and long contact time to extract at all, unlike the flavonoids and tannins in the bark, which dissolve into hot water almost immediately. Analyzed samples of brewed Pau d'Arco tea have come back with lapachol concentrations of just 0.05–0.08%. That's roughly 25–30 times lower than what's present in the raw bark.
For comparison, standardized commercial extracts (capsules and tinctures built specifically to concentrate the compound) are typically formulated to 2–3% lapachol. That's the difference between a compound present at trace levels in a water decoction and one deliberately concentrated far beyond what steeping bark in hot water will ever produce.
Keep that ratio in mind for everything below. Almost all the research that follows was done with isolated, purified lapachol, at doses nowhere close to what a cup of brewed tea delivers.
The rest of the bark
Lapachol isn't the only thing in Pau d'Arco bark, which is worth keeping in mind while reading the isolated-compound research below. The inner bark carries a broad mix of naphthoquinones, flavonoids, tannins, and other constituents beyond lapachol alone, as detailed in a 2009 ethnopharmacological review of Red Lapacho tea. Separate studies have identified specific antibacterial compounds in Tabebuia impetiginosa inner bark active against H. pylori and other intestinal bacteria, compounds distinct from lapachol itself.
Traditional preparation reflects that. It isn't a quick steep. It's a decoction: bark simmered in water for fifteen minutes or longer, sometimes repeated several times a day, a process built to draw out a wide mix of compounds rather than isolate one. That's a meaningfully different approach from the NCI trials below, which tested a single purified molecule on its own.
Whether the full mix of compounds behaves differently, or better, than lapachol alone hasn't been tested directly. No controlled trial has compared whole-bark decoction to isolated lapachol at matched doses. It's a reasonable question to raise given how different the two preparations are, but it's an open question, not an established finding.
The 1968–1970 NCI cancer trials
Lapachol's most-cited chapter starts with the National Cancer Institute. In 1968, based on strong activity against Walker 256 tumors in animal models (better than 90% tumor inhibition in early screening), lapachol moved into Phase I human trials.
It didn't go well. Patients receiving isolated lapachol developed nausea, vomiting, and a pronounced anticoagulant effect: anemia and abnormal bleeding tendency, traced back to lapachol's action as a potent inhibitor of vitamin K epoxide reductase and vitamin K quinone reductase, two enzymes the body needs to recycle vitamin K for normal blood clotting. Researchers couldn't find a dose that produced a therapeutic anti-tumor effect without also producing this toxicity. The trials were discontinued, and by the early 1970s the NCI's interest in lapachol as a systemic chemotherapy candidate had ended (Block et al., Cancer Chemotherapy Reports, 1974).
This is the finding that still gets cited, correctly, as the reason lapachol "failed" as a cancer drug. What usually gets left out is the dose. These trials used purified, isolated lapachol administered directly, not bark tea. The vitamin K–antagonist effect is a real, documented property of the isolated compound. Whether it's relevant at the concentrations present in a cup of brewed tea is a separate question the NCI trials weren't designed to answer.
What the animal toxicity studies found
A handful of studies through the 2000s looked at lapachol's reproductive and genetic toxicity in rats, and they're the ones most often cited when people ask if Pau d'Arco is dangerous:
- Fetal effects in pregnant rats. Felício et al. (2002) found that lapachol impaired fetal growth in pregnant rats at the doses tested.
- Embryolethality. Guerra et al. (2001) reported embryo loss in rats given lapachol during early pregnancy.
- Reproductive toxicity in males. da Silveira e Sá and de Oliveira Guerra (2007) found reduced seminal vesicle weight in male rats after short-term lapachol treatment.
- Genetic damage (clastogenicity). Maistro et al. (2010) documented chromosome-level damage in rats given lapachol.
These are legitimate, peer-reviewed findings, and they're the scientific basis for the standard caution that pregnant or nursing women should avoid Pau d'Arco. But every one of these studies used isolated lapachol dosed by body weight, typically 100 mg/kg or more, which doesn't translate directly to steeping bark in hot water. None of them tested brewed tea itself. That distinction matters for reading the results honestly. It doesn't erase the reason for the caution.
Antimicrobial and antifungal research
Away from the cancer and toxicity literature, lapachol has a long-standing reputation as an antimicrobial compound, and more recent work has tested that directly against drug-resistant organisms. Lapachol derivatives called thiosemicarbazones have shown strong activity against Paracoccidioides brasiliensis, a fungus resistant to standard antifungals, with minimum inhibitory concentrations as low as 0.01–0.10 µmol/mL in lab testing. Lapachol and its relatives have also shown broader antimicrobial and antiviral activity across studies going back decades.
Recent research (2024–2025)
Topical use for skin redness (2024). A study published in the Journal of Cosmetic Dermatology tested a topical serum containing lapachol on 25 adults with mild-to-moderate facial redness from rosacea, acne, or sun damage. Over eight weeks of twice-daily application, most participants saw measurable improvement, attributed to lapachol's anti-inflammatory properties. This is a topical cosmetic application. It says nothing about oral consumption, but it's a useful data point on lapachol's anti-inflammatory activity outside the cancer-research context.
Bladder cancer cell research (2025). A study in Scientific Reports tested isolated lapachol against bladder cancer cell lines in vitro and found dose- and time-dependent reductions in cell viability, migration, and colony-forming ability, along with markers consistent with apoptosis (programmed cell death). This is lab cell-culture research, not a clinical trial, and not evidence of an effect in humans. It's a sign that lapachol's anticancer activity, first flagged in the 1960s, is still being studied more than fifty years later.
Where does that leave the tea?
Put together, the research tells a consistent story. Lapachol is a bioactive compound with real pharmacological effects: anticoagulant, antimicrobial, antifungal, and toxic to certain cancer cell lines. All of that was demonstrated using isolated, concentrated doses far above what water extraction pulls from Pau d'Arco bark. The caution around pregnancy and high-dose supplementation is well-founded and worth taking seriously.
But the studies driving that caution were never testing the tea itself. And the gap between raw bark (1.5–2% lapachol) and brewed tea (0.05–0.08%) is too large to ignore in either direction. Tea is also a mixture of the bark's full compound profile, not a single isolated molecule, and none of the research above was designed to account for that difference. It doesn't make the tea risk-free, and it doesn't mean the tea carries the same risk profile as an isolated extract dosed at pharmacological levels.
For the practical side, how much is a reasonable daily amount and who should avoid it, see our safety and side effects guide.
Safety & side effects guide → Is Pau d'Arco tea toxic? → More Pau d'Arco research → Pau d'Arco & cancer research →
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