Oil of Oregano for Intestinal Parasites

Oil of oregano is widely sold for parasite cleanses, but the human evidence for intestinal parasites is one 2000 report of 14 adults with no control group (Force 2000). After six weeks of 600 mg of emulsified oil of oregano a day, Blastocystis hominis had disappeared from the stools in eight cases, Entamoeba hartmanni in all four cases and Endolimax nana in the one case (Force 2000).

Those results are weaker than they look. The CDC considers Entamoeba hartmanni and Endolimax nana nonpathogenic: neither causes symptoms (CDC, intestinal amebae). Whether Blastocystis causes disease is still debated (CDC, Blastocystis), and in a placebo-controlled trial it cleared without active treatment in 29% of children (Heyland 2012). Without a comparison group, the study can't separate an effect of the oil from that background.

Oregano oil's main active compound is carvacrol, a phenol also found in thyme; its antimicrobial activity is put down to its free hydroxyl group and its fat-solubility (Sharifi-Rad 2018). The same compound is behind oregano oil's use for SIBO.

In lab cultures, oil from Origanum virens slowed the growth of Giardia lamblia, reduced the parasites' ability to attach (p < 0.001) and, with the other phenol-rich oils tested, killed almost half of them over time, apparently by damaging their outer membrane (Machado 2010). The oils did not harm mammalian cells in the same experiments (Machado 2010).

An Origanum vulgare extract applied to Giardia cysts in a dish was not significantly different from metronidazole at its highest concentration (P > 0.05) (Davoodi 2018), and purified carvacrol killed cultured Blastocystis, at lethal concentrations that differed between isolates (Grabensteiner 2007).

These experiments put oil or extract directly on parasites. None shows that a swallowed capsule reaches those concentrations in the gut, and a review of carvacrol's effects found human trials still lacking (Sharifi-Rad 2018).

No dose of oregano oil has been shown to treat intestinal parasites. The only study used 600 mg of an emulsified oil of oregano daily for six weeks (Force 2000); the abstract does not give the carvacrol content or how the dose was split through the day.

Supplement contents often differ from their labels, so results from one product may not apply to another (LactMed). For comparison, giardiasis, which can cause diarrhoea (sometimes severe), weight loss and, in children, failure to thrive, has proven drug treatments: in a Cochrane review, metronidazole was usually given for five to 10 days, and albendazole 400 mg once daily for five to 10 days was about as effective for clearing the parasite (RR 0.99, 95% CI 0.95 to 1.03) (Granados 2012).

Force and colleagues gave 600 mg of emulsified oil of oregano daily for six weeks to 14 adults whose stools tested positive for Blastocystis hominis, Entamoeba hartmanni or Endolimax nana (Force 2000). Afterwards, E. hartmanni had gone in all four cases, E. nana in the one case and Blastocystis in eight cases, with lower Blastocystis scores in three more (Force 2000).

Gastrointestinal symptoms improved in 7 of the 11 people who had Blastocystis (Force 2000). The report runs to two journal pages and had no placebo or untreated group.

Blastocystis can clear, and symptoms can ease, without active treatment. In a double-blind trial in 37 children with recurrent abdominal pain, Blastocystis cleared spontaneously in 29% on placebo, and pain scores fell at least as much on placebo (7.4 to 3.0) as on co-trimoxazole (6.9 to 4.1), with no significant difference (Heyland 2012). In another trial in children with symptoms, 40% of untreated children had recovered clinically and 26.6% had cleared the cysts by day 15 (Dinleyici 2011). That trial also tested the probiotic yeast covered on our Saccharomyces boulardii for intestinal parasites page.

The standard drug's results are mixed. In the same children's trial, metronidazole cleared the cysts in 80% by day 15, against 26.6% with no treatment (p = 0.011) (Dinleyici 2011). But in 50 adults with persistent symptoms and only Blastocystis on microscopy, metronidazole did no better than placebo on stool consistency or any other symptom (Cobuccio 2023). Reviews report eradication rates with metronidazole anywhere from 0% to 100% (Batista 2017), and a 2025 systematic review questioned it as first-line treatment (Ulusan Bağcı 2025).

Against that background, an uncontrolled study can't show that the symptom improvements or clearance seen with oregano oil came from the oil.

There is no randomised trial of oregano oil for any intestinal parasite, no study in children, and no human study against Giardia, Entamoeba histolytica or worms. We found no replication of the 2000 study by any other group.

The 2000 study's abstract does not mention side effects (Force 2000). Oregano and oregano oil are generally recognised as safe as food ingredients, and gastrointestinal upset and allergic skin reactions are reported rarely (LactMed). In a one-month study of purified carvacrol at 1 or 2 mg/kg a day in healthy adults, all lab values stayed within normal ranges, although prothrombin time (a clotting test) rose at the lower dose (Ghorani 2021).

Systemic allergic reactions to oregano and thyme have been documented in one patient, who also reacted on skin testing to other mint-family herbs including marjoram, mint and sage (Benito 1996). Because no safety data exist, LactMed advises avoiding more than food amounts of oregano while breastfeeding (LactMed). We found no study of oregano oil supplements in pregnancy either, so ask your clinician before using it then.

No study has tested oregano oil alongside antiparasitic drugs. Purified carvacrol slightly raised prothrombin time, within the normal range, in healthy volunteers; whether that matters with a blood thinner is untested (Ghorani 2021).