Chlorella side effects

Chlorella's most commonly reported side effects are mild and digestive: nausea, diarrhea, abdominal cramping, flatulence and green-tinted stools. A government drug database describes chlorella as usually well tolerated at that level. Less common but more serious effects — severe allergic reactions, photosensitivity, and case reports involving the kidneys and liver — are documented too, and some of them are tied to specific circumstances like a contaminated supply or impaired kidney function rather than chlorella at any dose.

The common ones: digestive upset

A government reference database on chlorella describes it as usually well tolerated, with nausea, diarrhea, abdominal cramping, flatulence and green stools as the effects reported. Green stools specifically come from chlorella's chlorophyll content and aren't a sign anything is wrong on their own. None of the trials summarized in chlorella benefits: what the research does and doesn't show reported a dropout rate specifically tied to digestive side effects, which is a different kind of evidence than a formal tolerability trial would give — the trials we found were designed to measure cholesterol, blood pressure or exercise markers, not systematically track side effects.

Allergic reactions, including anaphylaxis

The same database reports severe allergic reactions, including anaphylaxis, in people taking chlorella and in people who prepare chlorella tablets for a living — an occupational exposure route, not just an oral one. A 2020 case report described a 75-year-old man who developed anaphylaxis (severe low blood pressure, wheeze, widespread hives and diarrhea) after taking Chlorella vulgaris supplements, the first reported case of anaphylaxis to that specific species; the report notes a small number of earlier case reports describe similar reactions to other Chlorella species. Photosensitivity reactions — unusual skin sensitivity to sunlight — have also been reported following chlorella ingestion.

Vitamin K and blood-thinning medication

Chlorella naturally contains vitamin K, and that's specifically relevant if you take warfarin or another blood-thinning medication: the same database notes chlorella's vitamin K content may reduce warfarin's effectiveness, and a 1995 case report described a 75-year-old man on stable, well-controlled warfarin therapy whose blood-clotting test values rose above the therapeutic range after he started taking chlorella, with no other cause identified. See chlorella and medications: what to check for what to ask a doctor before combining the two.

Rarer, more serious case reports

A few case reports describe more serious organ-specific effects, each tied to a specific circumstance rather than chlorella at any dose in anyone:

• Kidney inflammation in a child. A case report described a boy who developed acute tubulointerstitial nephritis — kidney inflammation found through a school health screening — after three months of taking chlorella tablets; his kidney function improved after stopping chlorella and starting steroid treatment. The report's authors suggested chlorella may have acted as an allergen triggering the injury in this case.
• Manganese buildup in a dialysis patient. A case report described a patient on long-term dialysis who developed Parkinson's-like symptoms after taking a chlorella extract that supplied 1.7 mg of manganese in its usual daily dose; brain imaging showed a manganese-related pattern that improved with chelation treatment. Dialysis patients don't clear manganese the way people with normal kidney function do, which is central to why this case happened.
• Liver injury tied to a contaminated supply. A case report described a lung cancer patient on chemotherapy who developed jaundice, appetite loss and lab evidence of acute liver injury after self-medicating with several supplements, including chlorella at 520 mg a day. Testing found that specific chlorella product was contaminated with cyanobacteria producing the toxin microcystin-LR — the researchers attributed the injury to an interaction between the chemotherapy drug, another herbal supplement, and the contaminated chlorella, not to chlorella alone. See how to choose a good chlorella for why contamination testing matters.

Questions people ask

What are the side effects of chlorella?

The most commonly reported effects are digestive: nausea, diarrhea, cramping, flatulence and green stools. Less commonly, allergic reactions including anaphylaxis, and photosensitivity, have been reported. Rare case reports describe more serious kidney, liver or neurological effects tied to specific circumstances — impaired kidney function, a contaminated product, or an allergic response.

Can chlorella cause diarrhea?

Yes, it's one of the digestive effects reported in the reference database above, alongside nausea, cramping, flatulence and green stools.

Does chlorella cause green stool?

Yes — it's a reported effect, attributed to chlorella's natural chlorophyll content, and isn't on its own a sign of a problem.

Can chlorella cause an allergic reaction?

Yes. Severe allergic reactions, including anaphylaxis, have been reported both in people taking chlorella and in people who prepare chlorella tablets occupationally. A 2020 case report describes a specific instance in detail.

Is chlorella safe for the kidneys?

We found no trial testing chlorella as beneficial for kidney function, and case reports exist in the other direction — a child's kidney inflammation linked to chlorella tablets, and manganese buildup in a dialysis patient specifically. See who should not take chlorella for who those cases mean should ask a doctor first.