Guaco (Mikania glomerata)
Guaco syrup is one of Brazil's most popular cough remedies and is even supplied by the public health system, yet not a single controlled trial has tested whether it works. The case rests on lab studies of its coumarin, and a human study found that the usual syrup dose does not deliver enough of it to open the airways.
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What the science says
- Relieves cough and acts as an expectorant Limited evidence
A 2025 review that mapped 57 studies of guaco found 23 biological activities for Mikania glomerata, all in lab and animal work, and no clinical evidence for its popular uses as an expectorant, against fever or for joint pain. There are no randomized trials of guaco for cough.
- Bronchodilator effect Limited evidence
Guaco extracts relaxed strips of human bronchi and guinea-pig trachea in the lab, an effect linked to coumarin. But in a human study, a single 60 mL dose of guaco syrup did not produce enough coumarin in the blood, and the authors concluded that the recommended syrup dose does not reach levels that would open the airways.
- Safe at usual doses Limited evidence
A small phase I trial in 19 healthy volunteers who took guaco oral solution twice daily for 2 weeks, at up to 30 mL per dose, found no adverse events attributable to it and blood tests stayed within normal ranges. That is reassuring for short-term use but says nothing about whether it works.
Dosage & safety
How to take it
Interactions
With medications
- anticoagulants (warfarin and others)
Brazilian package leaflets advise against the combination because guaco contains coumarin; there are no interaction studies. avoid guaco unless your doctor agrees
- cough suppressants (dextromethorphan, codeine)
Guaco is used as an expectorant, so suppressing the cough at the same time works against it. do not combine for a productive cough
With other supplements
- other guaco syrups or teas
Adds up the coumarin dose, which can harm the liver at high doses. use one product at the label dose
Well-established interactions only — this is not a complete list. Always tell your doctor and pharmacist what you take.
Frequently asked questions
Does guaco syrup work for cough?
There is no proof: no controlled trial has tested guaco for cough, and a 2025 review of 57 studies found only lab and animal evidence. The evidence for guaco is limited.
Does guaco open the airways?
Guaco extracts relaxed human bronchial tissue in the lab, but in a human study a single 60 mL dose of guaco syrup did not produce enough coumarin in the blood to have that effect.
Is guaco safe?
In a small trial, 19 healthy volunteers took guaco for 2 weeks without adverse events attributable to it. Guaco contains coumarin, which can harm the liver at high doses, so stay at the label dose and avoid it with anticoagulants and in pregnancy.
Can children take guaco syrup?
Brazilian package leaflets advise against guaco in young children below the age on the label. A child's cough with fever, wheezing or shortness of breath needs a doctor, not a syrup.
Scientific references
- Biological Activities of Mikania glomerata and Mikania laevigata: A Scoping Review and Evidence Gap Mapping
- A kinetic study of the main guaco metabolites using syrup formulation and the identification of an alternative route of coumarin metabolism in humans
- Safety evaluation of Mikania glomerata and Mikania laevigata in healthy volunteers: A randomized, open label and multiple dose phase I clinical trial
- Bronchodilator activity of Mikania glomerata Sprengel on human bronchi and guinea-pig trachea
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