Spirulina Side Effects

Spirulina is often presented as a “superfood” without any drawbacks. In reality the safety profile of this cyanobacterium is generally favorable, but adverse reactions do exist — and most of them are linked not so much to spirulina itself as to the quality of the raw material, individual sensitivity, and combination with medications. The editors have organized what is known about undesirable effects from clinical data and safety reviews.

How safe is spirulina overall

Spirulina (the genusArthrospira, primarilyA. platensisandA. maxima) is a cyanobacterium that people have consumed for centuries: there is historical evidence of its use as food near Lake Chad in Africa and in the Valley of Mexico. A long history of dietary use is not proof of absolute safety, but it does indicate that under normal circumstances serious toxic effects are rare.

The most thorough safety assessment was conducted by the United States Pharmacopeia (USP) in 2011. Experts analyzed clinical studies, reports of adverse reactions, and animal toxicology data and concluded that spirulina in itself does not pose serious health risks, so a pharmacopeial monograph with mandatory requirements for contaminant control could be developed for it.

The USP's key caveat concerned contamination specifically: cyanobacteria are grown in open ponds, where other, toxin-producing algae can get into the culture, along with heavy metals from the water or soil. That is why the question “is spirulina safe” almost always comes down to the question “is a specific batch from a specific manufacturer safe.”

In clinical studies where participants received several grams of spirulina per day for weeks and months, the incidence of side effects was generally low and mostly limited to mild gastrointestinal complaints. Systematic reviews, in particular Karkos et al. (2011), found no signals of serious toxicity at standard doses.

The most common adverse reactions

The most common group of undesirable effects is gastrointestinal. Spirulina contains a lot of protein and pigments, as well as a distinctive smell and taste, so in the first days of use some people notice nausea, bloating, changes in bowel movements, or mild abdominal discomfort. These symptoms usually disappear on their own or when the serving is reduced.

A characteristic and entirely harmless feature is a change in stool color to dark green due to chlorophyll and phycocyanin. This is not a sign of bleeding or liver problems, but people who are unaware of the effect are sometimes frightened by it.

The second group is general sensations: headache, mild dizziness, a feeling of “heaviness” in the first days of use. There are no reliable data on the mechanism of such complaints, and popular explanations such as a “detox crisis” have no scientific basis. If the symptoms are persistent, it is more sensible to stop taking it than to “push through.”

Below is a summary table of the reactions described in the literature. The editors present frequency qualitatively, since the exact percentages vary between studies and depend on the dose, form, and population.

Allergy and immune reactions

Like any protein-rich product, spirulina can cause allergy. Cases of urticaria, itching, and even anaphylaxis after taking cyanobacteria supplements have been described. Among the potential allergens, phycocyanin and other proteins are considered. People with a burdened allergy history should start with a small serving and remember the possibility of cross-sensitivity.

A separate topic is immunostimulating properties. In laboratory and some clinical work, spirulina enhanced the activity of natural killer cells and the production of cytokines. For a healthy person this is neutral or even desirable, but for patients with autoimmune diseases the situation is different.

Lee and Werth (2004) described patients in whom an exacerbation of autoimmune skin diseases (dermatomyositis, pemphigus) coincided in time with the intake of immunostimulating supplements, including spirulina and chlorella. A cause-and-effect relationship cannot be proven in such case series, but it is a sufficient signal for caution.

The editors' practical conclusion is simple: if you have been diagnosed with lupus, rheumatoid arthritis, multiple sclerosis, psoriasis, or another autoimmune disease, or if you take immunosuppressants, the decision to take spirulina should be made together with your treating physician.

Contamination: microcystins, metals, bacteria

The most serious risks associated with spirulina in the literature concern not theArthrospiraitself, but impurities. Spirulina is not among the typical producers of microcystins, but when grown in open bodies of water, toxic cyanobacteria such asMicrocystis.

Microcystins are hepatotoxins that inhibit phosphatases in liver cells. A study by Gilroy et al. (2000) in Oregon found them in a significant proportion of blue-green algae products, mostly fromAphanizomenon flos-aquae. Later, Roy-Lachapelle et al. (2017) analyzed various algae supplements and also found cyanotoxins in some samples, although spirulina products were contaminated less often.

The second type of contamination is heavy metals: lead, cadmium, mercury, arsenic. Microalgae actively bind metals from the environment, so the quality of the water and fertilizers is critically important. The third is microbiological: violations of drying and storage conditions can lead to the growth of bacteria and mold.

It is precisely possible contamination that is linked to the isolated reports of serious reactions: hepatotoxicity (Iwasa et al., 2002) and rhabdomyolysis (Mazokopakis et al., 2008). In both cases the authors could not unambiguously separate the effect of spirulina from that of impurities, but such reports serve as a reminder that “natural” does not mean “safe by default.”

Interactions and how to minimize the risks

Spirulina contains a noticeable amount of vitamin K, though less than green leafy vegetables or chlorella. For people taking warfarin or other vitamin K antagonists, any regular dietary changes that affect the intake of this vitamin can alter the INR. So intake should be coordinated with a doctor and clotting parameters should be monitored.

Theoretically, the immunostimulating effect may conflict with immunosuppressive therapy (after transplantation, in autoimmune conditions). Spirulina also contains phenylalanine, so it is contraindicated in people with phenylketonuria. Its high protein and purine content calls for caution in gout and severe renal insufficiency.

To reduce the risks, the editors recommend following a few simple rules:

• choose products with a certificate of analysis (CoA) for microcystins, heavy metals, and microbiology;
• give preference to manufacturers that operate under GMP standards and undergo independent testing;
• start with a small serving (1–2 g per day) and increase gradually;
• stop taking it if a rash, pain in the right upper abdomen, dark urine, or severe muscle weakness appears;
• do not combine it with anticoagulants and immunosuppressants without a doctor's approval.

These measures do not guarantee 100% safety, but they substantially reduce the likelihood of the most serious scenarios described in the literature.

Editors' conclusions

According to clinical studies and the USP assessment, spirulina has a favorable safety profile: the most common side effects are mild gastrointestinal complaints and a change in stool color.

The real serious risks are related mainly to the quality of the raw material — contamination with microcystins, heavy metals, or microorganisms. That is why choosing a trusted manufacturer matters more than the marketing promises on the label.

Special caution is needed for people with autoimmune diseases, phenylketonuria, gout, and those taking anticoagulants or immunosuppressants.

If the topic interests you, we also recommend reading our articles “Who Should Not Take Spirulina,” “Spirulina: Product Forms and Which to Choose,” and “Chlorella Side Effects.”

References

1. Marles RJ, Barrett ML, Barnes J, et al. United States Pharmacopeia safety evaluation of Spirulina. Crit Rev Food Sci Nutr. 2011;51(7):593–604.
2. Karkos PD, Leong SC, Karkos CD, Sivaji N, Assimakopoulos DA. Spirulina in clinical practice: evidence-based human applications. Evid Based Complement Alternat Med. 2011;2011:531053.
3. Gilroy DJ, Kauffman KW, Hall RA, Huang X, Chu FS. Assessing potential health risks from microcystin toxins in blue-green algae dietary supplements. Environ Health Perspect. 2000;108(5):435–439.
4. Roy-Lachapelle A, Solliec M, Bouchard MF, Sauvé S. Detection of cyanotoxins in algae dietary supplements. Toxins (Basel). 2017;9(3):76.
5. Lee AN, Werth VP. Activation of autoimmunity following use of immunostimulatory herbal supplements. Arch Dermatol. 2004;140(6):723–727.
6. Iwasa M, Yamamoto M, Tanaka Y, Kaito M, Adachi Y. Spirulina-associated hepatotoxicity. Am J Gastroenterol. 2002;97(12):3212–3213.
7. Mazokopakis EE, Karefilakis CM, Tsartsalis AN, Milkas AN, Ganotakis ES. Acute rhabdomyolysis caused by Spirulina (Arthrospira platensis). Phytomedicine. 2008;15(6–7):525–527.