Chlorella Side Effects: What to Know Before You Start
Chlorella, a single-celled freshwater alga, is marketed in tablets and powders as a protein-rich ‘superfood’ and a ‘detox’ aid. It is a food supplement in the UK rather than a medicine, so its packaging rarely mentions side effects. Yet it does cause problems for some people, and it can interact with prescribed drugs. Our editorial team looked at what the evidence actually documents.
What chlorella contains
The most common species in supplements are Chlorella vulgaris and Chlorella pyrenoidosa. Dried chlorella is roughly half protein by weight and contains chlorophyll, carotenoids, iron, vitamin K and a range of B vitamins. Because the algal cell wall is tough, manufacturers usually ‘crack’ it to improve digestibility.
Several of these components explain the side-effect profile. High chlorophyll content colours the stool, vitamin K affects blood clotting, iron can irritate the gut and the cell wall fibre can ferment in the colon.
Chlorella is often promoted as a source of vitamin B12. According to work by Watanabe and colleagues, some chlorella products do contain active B12, but levels vary widely and some contain inactive analogues. Vegans should not rely on it as their sole B12 source.
Human studies, reviewed by Bito and colleagues and by Panahi and colleagues, are mostly small and short. They point to possible modest benefits for blood lipids, blood pressure or immune markers, but safety data over years of use are limited.
Digestive side effects
The most common complaints are digestive. People new to chlorella frequently report bloating, flatulence, nausea, stomach cramps or looser stools. Green-coloured faeces are expected and harmless, but can alarm those who were not warned.
These effects usually reflect a sudden increase in fibre and plant material and tend to settle within one to two weeks. Starting with a small amount, for example one or two grams a day, and increasing gradually reduces discomfort.
Taking chlorella with meals rather than on an empty stomach and drinking enough water also helps. If symptoms are severe, include vomiting or blood in the stool, or do not settle, stop the supplement and seek medical advice.
People with irritable bowel syndrome or inflammatory bowel disease may be more sensitive to fermentable fibre and should be particularly cautious when trying algae-based products.
Allergy and sun sensitivity
Allergic reactions to chlorella are uncommon but documented, ranging from skin rashes to breathing difficulties and, rarely, anaphylaxis. People who react to other algae products, or who have multiple food allergies, should be careful with a first dose.
Photosensitivity is a less familiar risk. In the 1970s, cases of skin reactions on sun-exposed areas were reported in Japan after consumption of chlorella products that contained high levels of pheophorbide, a breakdown product of chlorophyll. Modern manufacturing standards limit pheophorbide, but quality differs between suppliers.
Anyone who notices unusual redness, swelling or burning on sun-exposed skin after starting chlorella should stop taking it and consult a doctor. This is especially relevant for people already using medicines that cause photosensitivity, such as certain antibiotics or acne treatments.
Signs that warrant urgent help include:
- swelling of the lips, tongue or throat;
- wheezing or difficulty breathing;
- widespread hives;
- dizziness or collapse.
Interactions with medicines
The interaction with the greatest clinical significance is with warfarin and other vitamin K antagonists. Chlorella can contain substantial amounts of vitamin K, which counteracts warfarin. A case report by Ohkawa and colleagues described reduced warfarin effect in a patient taking chlorella.
| Medicine or group | Potential issue | Practical step |
|---|---|---|
| Warfarin | Vitamin K may lower INR and anticoagulant effect | Avoid, or only with anticoagulation clinic advice |
| Immunosuppressants | Possible immune-stimulating effects | Discuss with the transplant or specialist team |
| Photosensitising drugs | Additive sun sensitivity | Avoid strong sun exposure, monitor skin |
| Iron supplements | Extra iron and gut irritation | Check total intake |
Direct-acting anticoagulants such as apixaban or rivaroxaban are not vitamin K dependent in the same way, but it is still sensible to tell your pharmacist about any supplement taken alongside them.
Studies such as that by Kwak and colleagues suggest chlorella may modestly influence immune activity. That is marketed as a benefit, but it is a reason for caution in people with autoimmune disease or those taking medicines that deliberately suppress the immune system.
Quality, contamination and who should avoid it
Algae grown in open ponds can absorb heavy metals and other contaminants from their environment. Reputable brands test for lead, cadmium, mercury and arsenic and publish certificates of analysis. Cheap products without batch testing carry more uncertainty.
Chlorella is sometimes confused with spirulina, which is a cyanobacterium rather than a true alga. Blue-green algae products harvested from the wild have been linked to microcystin toxins, so the source matters for both.
Groups who should avoid chlorella or seek medical advice first include pregnant and breastfeeding women, people taking warfarin, those with an iodine or algae allergy history, people on immunosuppressants and anyone with a condition that affects iron metabolism, such as haemochromatosis.
For athletes, chlorella offers no proven performance benefit. If you are tested under UK Anti-Doping rules, choose batch-tested products to reduce the risk of contamination with prohibited substances, which can affect any supplement category.
Editorial conclusions
Chlorella is a food supplement with a generally mild side-effect profile, dominated by temporary digestive symptoms.
Less common but important risks include allergy, photosensitivity from poorly processed products and a clinically relevant interaction with warfarin through vitamin K.
Choosing a tested product, starting with a small dose and checking with a pharmacist if you take regular medicines reduces most of the risk.
You may also be interested in our articles on how to take ginseng, on who should avoid synephrine and on blood tests during N-acetylcysteine supplementation.
References
- Bito T, Okumura E, Fujishima M, Watanabe F. Potential of Chlorella as a dietary supplement to promote human health. Nutrients. 2020;12(9):2524.
- Panahi Y, Darvishi B, Jowzi N, Beiraghdar F, Sahebkar A. Chlorella vulgaris: a multifunctional dietary supplement with diverse medicinal properties. Curr Pharm Des. 2016;22(2):164–173.
- Merchant RE, Andre CA. A review of recent clinical trials of the nutritional supplement Chlorella pyrenoidosa in the treatment of fibromyalgia, hypertension, and ulcerative colitis. Altern Ther Health Med. 2001;7(3):79–91.
- Watanabe F. Vitamin B12 sources and bioavailability. Exp Biol Med (Maywood). 2007;232(10):1266–1274.
- Kwak JH, Baek SH, Woo Y, et al. Beneficial immunostimulatory effect of short-term Chlorella supplementation: enhancement of natural killer cell activity and early inflammatory response (randomized, double-blinded, placebo-controlled trial). Nutr J. 2012;11:53.
- Ohkawa S, Yoneda Y, Ohsumi Y, et al. Warfarin therapy and chlorella. Rinsho Shinkeigaku. 1995;35(7):806–807.
- https://steroidsbuy-online.com/
