Saccharomyces Boulardii for Intestinal Parasites

Saccharomyces boulardii is a probiotic yeast with good evidence for some kinds of diarrhoea (see S. boulardii for diarrhoea). For intestinal parasites, the evidence is much thinner: five small trials, all in protozoal infections (single-celled parasites such as Giardia, Entamoeba histolytica and Blastocystis), and four of them used the yeast as an add-on to metronidazole rather than on its own.

When added to standard treatment, results were mostly positive: no Giardia cysts at follow-up in 30 adults versus 6 of 35 controls, and diarrhoea that settled 30-36 hours sooner in two amoebiasis trials. But a third amoebiasis trial in 85 children found no difference, and there is no reliable evidence for worms. The honest reading is a possible add-on for protozoal infections, not a parasite treatment.

S. boulardii doesn't appear to kill parasites directly. In a lab study, live yeast and fluids from it cut the share of Entamoeba histolytica that attached to human red blood cells from about 70% to 32%, which the authors put down to yeast substances competing for the amoeba's binding sites (Rigothier 1994). In young rats with caecal amoebiasis, the yeast reduced disease severity, yet it had no amoeba-killing action in a dish (Rigothier 1990).

For Giardia, an animal study found that S. boulardii, alone or with metronidazole, cut the parasite load by about 90% and protected the gut lining from damage and oxidative stress caused by both the parasite and the drug (Ribeiro 2021).

In rats infected with Blastocystis subtype 3, live S. boulardii improved the colon tissue and lowered inflammatory signals (IL-6, IL-8, TNF-alpha and iNOS); given with metronidazole, it cleared all parasite stages from stool and gut washings (Méabed 2019).

These are lab and animal results. They give a plausible reason the yeast might help alongside drugs, but they can't show that it shortens illness or prevents relapse in people, or which parasites respond. Only the human trials below can answer that.

The four trials that reported a dose used 250 mg of S. boulardii, twice daily in most and three times daily in the adult amoebiasis trial, which used freeze-dried (lyophilised) capsules. In the add-on trials it was taken for 7-10 days alongside the antiparasitic course, not instead of it.

The trial abstracts don't report the number of live yeast cells per capsule, so products with the same milligram dose may not be equivalent. Because S. boulardii is a yeast, antifungal medicines may reduce its effect, so avoid taking it during antifungal treatment unless your prescriber advises otherwise.

In a double-blind, placebo-controlled trial, 65 adults with giardiasis took metronidazole 750 mg three times daily for 10 days, with either S. boulardii 250 mg twice daily (n=30) or empty capsules (n=35). At 2 weeks, Giardia cysts were found in 6 of the placebo group (17.1%) and none of the yeast group, and the same 6 were still positive at 4 weeks (Besirbellioglu 2006).

The abstract doesn't say how patients were allocated and reports no symptom outcomes, so this shows faster or more complete cyst clearance, not that people felt better sooner. For context, standard drugs such as metronidazole, tinidazole, nitazoxanide and albendazole are effective on their own (CDC; Granados 2012).

In 54 adults with acute amoebic dysentery, adding S. boulardii 250 mg three times daily to metronidazole plus iodoquinol cut diarrhoea from 48.0 to 12.0 hours (P<0.0001) and fever and abdominal pain from 24 to 12 hours; at 4 weeks, 18.5% of the drug-only group still passed cysts versus none with the yeast (P<0.02) (Mansour-Ghanaei 2003).

In an open-label trial of 50 children with amoebic bloody diarrhoea, the yeast shortened bloody diarrhoea from 72.0 to 42.2 hours (P<0.001) and cleared cysts faster, though all children had recovered and were cyst-free by day 10 (Dinleyici 2009). A separate trial in 85 children found no difference: median diarrhoea lasted 5 days without and 4.5 days with the yeast (p=0.965), and bloody diarrhoea, fever and pain were similar (Savaş-Erdeve 2009).

Standard care for amoebiasis is a nitroimidazole such as metronidazole followed by a second drug, such as paromomycin or iodoquinol, because metronidazole alone doesn't clear cysts from the bowel (Merck Manual). Notably, the adult trial's control group already took iodoquinol and 18.5% still passed cysts.

In 48 children with Blastocystis and gut symptoms for over 2 weeks, allocated alternately to S. boulardii, metronidazole or no treatment, clinical cure at day 15 was 77.7%, 66.6% and 40% (S. boulardii vs no treatment significant), and cysts had cleared in 72.2%, 80% and 26.6%. By day 30, clinical cure was 94.4% with the yeast and 73.3% with metronidazole (not a significant difference), and parasite clearance was about the same at 94.4% and 93.3% (Dinleyici 2011).

Alternating allocation isn't true randomisation, the groups were tiny, and Blastocystis often causes no symptoms, so whether it needs treating at all is debated.

No controlled trial with reported clinical results has tested S. boulardii for worms such as pinworm, roundworm or tapeworm, or for Cryptosporidium. One study in 90 patients with giardiasis, roundworm or both reported better gut bacteria and claimed better treatment results when an S. boulardii probiotic was added, but its abstract gives no clinical figures and doesn't describe randomisation (Pryshliak 2022).

All five controlled trials come from Turkey and Iran, two were run by the same research group, and none enrolled more than 85 people. For worm infections, papaya seed has been tested in two small trials, and oil of oregano only in an uncontrolled study; neither replaces prescribed medicine.

In the parasite trials S. boulardii was well tolerated; the larger children's amoebiasis trial recorded no side effects (Savaş-Erdeve 2009). Probiotics are unlikely to harm healthy people, according to the NIH Office of Dietary Supplements, but S. boulardii is a live yeast, and in rare cases it gets into the bloodstream (ODS).

In 5 Finnish university hospitals, at least 43% of patients with Saccharomyces bloodstream infection were taking an S. boulardii probiotic, an odds ratio of 14 compared with other bloodstream infections (Rannikko 2021). A systematic review of 108 cases found the main risk factors were intensive care, tube or intravenous feeding, diarrhoea and diabetes, and 36.1% of all patients died (Vinayagamoorthy 2023).

The main interaction is with antifungal medicines, which may stop the yeast from working; this is theoretical rather than tested (Drugs.com). Treatments that weaken the immune system matter for a different reason: they raise the risk of a yeast bloodstream infection (ODS).