Chlorella Side Effects
Chlorella has a reputation as a gentle and safe supplement, and for most healthy people this is true. However, it has specific side effects that spirulina does not — primarily related to chlorophyll, vitamin K, and the rigid cell wall. The editors have compiled the known adverse reactions and explained how to avoid them.
General safety profile
Chlorella has been sold for decades in Japan, Korea, and Taiwan as a mass-market dietary supplement, and over this time considerable experience of its use has accumulated. In the clinical studies reviewed by Bito et al. (2020) and Panahi et al. (2016), serious side effects occurred rarely.
However, the limitations must be honestly acknowledged: most studies lasted from a few weeks to a few months and included a relatively small number of participants. Rare reactions may not appear in such work, so some of the knowledge about safety comes from descriptions of individual clinical cases.
Chlorella's side effects can be roughly divided into four groups: gastrointestinal, dermatological (photosensitivity and allergy), drug interactions, and risks related to product quality.
Below is a summary in which the editors present frequency qualitatively, without invented percentages.
| Reaction | Approximate frequency | Mechanism or cause |
|---|---|---|
| Bloating, flatulence, nausea | Uncommon, at the start | Fiber, an unfamiliar product |
| Green stool color | Common | Chlorophyll |
| Photosensitivity | Rare | Pheophorbides — chlorophyll breakdown products |
| Allergic reactions | Rare | Algal proteins, including from inhaling dust |
| Change in the action of warfarin | Depends on the therapy | Vitamin K |
| Reactions due to contamination | Unknown | Metals, microbes in poor-quality batches |
Gastrointestinal reactions
The most common complaint is bloating and increased gas production in the first days of use. Chlorella contains fiber and cell wall polysaccharides, which are partly fermented by the gut microbiota.
Nausea, loose stools or, conversely, constipation, and a feeling of heaviness in the stomach are also possible. As a rule, the symptoms weaken after 1–2 weeks, when the intestine adapts.
Green stool color is an expected and harmless effect of the pigments, not a sign of pathology. However, black tarry stool is a different situation that requires immediate medical attention regardless of supplements.
To reduce gastrointestinal reactions, it is worth starting with a small dose, taking chlorella with food, and washing it down with enough water. If the symptoms do not pass, it is better to stop taking it.
Photosensitivity and allergy
A risk specific to chlorella is photosensitivity. When chlorophyll breaks down, pheophorbides are formed — compounds capable of absorbing light and triggering phototoxic reactions in the skin. There are historical cases in Japan when improperly processed chlorella products caused a rash and inflammation on exposed areas of skin after sun exposure.
After that, control of pheophorbide content in chlorella products was introduced in Japan. Quality manufacturers note compliance with these requirements, so the risk is associated primarily with poor-quality or improperly stored raw material.
Allergic reactions to chlorella have been described both with oral intake and in workers who come into contact with the powder (asthma, rhinitis). Symptoms may include rash, itching, and in severe cases difficulty breathing.
Like spirulina, chlorella has immunostimulating properties. Lee and Werth (2004) reported cases of exacerbation of autoimmune skin diseases with the intake of similar supplements, including chlorella, so people with autoimmune diseases need a doctor's consultation.
- rash, redness on exposed areas of skin after the sun;
- itching, urticaria, swelling;
- cough or difficulty breathing;
- exacerbation of a known autoimmune disease.
Drug interactions and iodine
Chlorella is one of the richest supplements in vitamin K. For people taking warfarin or other vitamin K antagonists, this is the most important caveat: a regular intake of vitamin K can reduce the effectiveness of the anticoagulant and increase the risk of thrombosis. Cases of a reduced INR while taking chlorella have been described in the literature.
Chlorella intake by patients on warfarin is possible only after a doctor's approval and with regular INR monitoring. You must not start or abruptly stop taking it on your own.
Chlorella may contain iodine, although as a rule much less than seaweed such as kelp. However, people with thyroid diseases, especially during therapy, should take into account the total iodine intake from all supplements.
Theoretical interactions are possible with immunosuppressants (through immune stimulation) and with photosensitizing drugs, for example some antibiotics or retinoids — the risk of skin reactions in the sun may accumulate.
Product quality and risk prevention
Some of chlorella's risks are related to quality: heavy metals from water or fertilizers, microbiological contamination, a high content of pheophorbides in improperly dried or stored raw material.
Effective ways to minimize the risks: choose products from manufacturers with GMP and independent testing, demand a certificate of analysis, check that the cell wall is broken, and store the supplement in a dark, dry place.
Start with a small dose and increase it gradually. Avoid prolonged intense sun exposure in the first weeks of use if you have sensitive skin.
Stop taking it immediately and see a doctor if a rash after the sun, facial swelling, difficulty breathing, yellowing of the skin, or dark urine appears.
Editors' conclusions
Chlorella is well tolerated by most healthy people; the most common side effects are bloating, nausea, and green stool color, which usually pass within 1–2 weeks.
Chlorella's specific risks are photosensitivity due to pheophorbides and a high proportion of vitamin K, critical for patients on warfarin.
People with autoimmune diseases, thyroid diseases, and those who take medication regularly should coordinate intake with a doctor.
For the full picture, read our articles “Chlorella: What It Is and How It Works,” “How to Take Chlorella: Dosage, Timing, Duration,” and “Spirulina Side Effects.”
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.
- Lee AN, Werth VP. Activation of autoimmunity following use of immunostimulatory herbal supplements. Arch Dermatol. 2004;140(6):723–727.
- 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.
- 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.
- Kay RA. Microalgae as food and supplement. Crit Rev Food Sci Nutr. 1991;30(6):555–573.
Andriy Melnyk
A strength-sports coach and author of programs for beginner and intermediate levels. Writes about training planning.
