Two studies of the same product category reached opposite conclusions, and the gap between them is the entire safety story. A Canadian survey of 18 commercial algae supplements found eight of them carrying cyanotoxins above tolerable daily intake levels. A French study pooling 623 dried spirulina samples from 95 producers found microcystin levels within safe regulatory limits throughout. Spirulina is not intrinsically contaminated or intrinsically clean — it is whatever the cultivation conditions and the testing regime made it.
Spirulina itself does not produce toxins. The risk is contamination by other cyanobacteria sharing the water, which can produce hepatotoxic microcystins, anatoxin-a, cylindrospermopsin, saxitoxin and BMAA. In an analysis of 18 commercial products containing spirulina or Aphanizomenon flos-aquae, 8 contained cyanotoxins above tolerable daily intake values. In contrast, 623 dried samples from controlled French small-scale production stayed within safe regulatory levels. Beyond contamination, three groups should not take it: people with phenylketonuria, because spirulina is 60–70% protein and rich in phenylalanine; people with autoimmune conditions, because its documented activity is immunostimulatory; and anyone who has reacted to it — anaphylaxis on first exposure is documented. There is no established safe dose in pregnancy.
This is a supplement where the manufacturer matters more than the ingredient, which is an unusual thing to be able to say and worth saying plainly.
Where the Toxins Come From
Not from spirulina. Arthrospira platensis and Limnospira species are not microcystin producers. The problem is that they are grown in open water, and open water contains other cyanobacteria.
Roy-Lachapelle's team put it bluntly: these products generally contain non-toxic cyanobacteria, but cultivation in natural waters without appropriate quality controls allows contamination by toxin-producing species present in the environment.
Their analysis of 18 commercially available products screened for total microcystins, seven individual microcystin variants, anatoxin-a and two derivatives, cylindrospermopsin, saxitoxin and β-methylamino-L-alanine. Eight of the 18 contained cyanotoxins at levels exceeding tolerable daily intake values.
Note what that sample contained: products based on spirulina and on Aphanizomenon flos-aquae, the blue-green algae harvested from natural lakes, which carries a higher contamination profile than controlled-pond spirulina. The headline figure covers both.
The Study That Shows It Is Fixable
Pinchart's 2023 analysis is the other half of the picture and it deserves equal weight. France has roughly 180 small-scale spirulina farms; the authors collected microcystin analyses carried out between 2013 and 2021 from 95 of them.
That amounts to 623 dried spirulina samples and 105 culture samples assayed by ELISA, with suspect samples re-examined by mass spectrometry. The conclusion: French production stayed within the safe regulatory level for microcystins throughout.
They did find contaminating cyanobacteria — 14 taxa across 539 count results — and mapped their prevalence and spread, which is precisely how a production problem gets managed rather than ignored.
Contamination is therefore a controllable variable. A buyer cannot control it, but a buyer can insist on seeing that somebody did.
Phenylketonuria: A Hard Contraindication
Spirulina is 60–70% protein by dry weight, which is most of why people buy it, and that protein contains phenylalanine like any other.
Phenylketonuria is a genetic disorder in which phenylalanine cannot be metabolised normally, and its entire management is dietary restriction. A concentrated protein powder is therefore contraindicated, and this is one of the few absolute exclusions in the whole supplement field rather than a cautious suggestion.
Anyone managing PKU already knows to read protein content. It is worth stating because spirulina is frequently filed mentally as "a green powder" rather than as a protein source.
Autoimmune Conditions and Immunosuppression
Spirulina's documented biological activity is immunostimulatory. That is the basis of most of the benefit claims made for it.
In autoimmune disease the immune system is already attacking the body's own tissue, and stimulating it further is the wrong direction of travel. The same logic applies to anyone taking immunosuppressive medication after a transplant or for an inflammatory condition, where the therapeutic goal is deliberate suppression.
There are no trials establishing harm here, and it would be dishonest to claim otherwise. The argument is mechanistic and the same one we apply across this category in our article on immunomodulation and autoimmune conditions: when a product's mechanism opposes your treatment's mechanism, the absence of a trial is not reassurance.
Allergy and First-Dose Reactions
Le and colleagues documented anaphylaxis in a 17-year-old male on the first occasion he took a spirulina tablet, confirmed by skin prick testing against the tablet's ingredients.
First-exposure anaphylaxis is possible where prior sensitisation happened through another route, and it means that having never taken spirulina before is not protective. Any breathing, throat or swallowing symptom after a dose is an emergency rather than a tolerance issue.
Who Should Avoid It
| Group | Reason | Strength of concern |
|---|---|---|
| Phenylketonuria | High phenylalanine in a 60–70% protein product | Absolute contraindication |
| Autoimmune disease | Documented immunostimulatory activity | Mechanistic; avoid without clinical advice |
| On immunosuppressants | Opposes the therapeutic goal | Mechanistic; discuss first |
| Known algae or seafood allergy | Anaphylaxis documented on first dose | Case-report level, but severe |
| Pregnancy and breastfeeding | No established safe dose; contamination variable | Insufficient data |
| Children | Same contamination uncertainty, smaller body mass | Insufficient data |
| Liver disease | Microcystins are specifically hepatotoxic | Applies only to contaminated product |
The Certificate That Actually Matters
Most supplement certificates of analysis report heavy metals and microbiology. For spirulina, that is testing for the wrong thing.
The contaminant class that matters here is cyanotoxins, and microcystin-LR is the one to look for by name. A heavy-metals panel proves nothing about microcystins, because they are unrelated compounds detected by entirely different methods — ELISA screening and mass spectrometry confirmation, as used in the French dataset.
So the question to a spirulina seller is specific: has this batch been assayed for microcystins, and what was the result? Our certificate of analysis guide covers how to read the answer, and our spirulina quality guide covers what else belongs on the document.
Common Mild Effects
The ordinary complaints are digestive and mostly dose-related: nausea, loose stools, bloating and cramping in the first days. Green discoloration of stool is expected and harmless. Headache is reported.
One genuinely benign oddity worth knowing: heavy intake of carotenoid-rich supplements can tint the palms and soles orange, which has produced more than one alarmed case report and resolves on reduction.
None of this is the reason to be careful about spirulina. The reason is the contamination question and the three exclusion groups above.
What to Do
- Ask for a microcystin assay on the batch you are buying. Not a heavy-metals panel, not a generic "tested for purity" claim — microcystin-LR by name.
- Prefer controlled cultivation to wild-harvested algae. The failure rate in the 18-product survey included lake-harvested Aphanizomenon, which is the higher-risk source.
- If you have phenylketonuria, do not take it. This is a dietary protein source, and that is the whole management of the condition.
- If you have an autoimmune condition or take immunosuppressants, ask first. The mechanism runs against your treatment.
- Start low and separately from other new supplements. First-dose anaphylaxis is documented, and you want to know what caused any reaction.
- Skip it in pregnancy. There is no established safe dose and the contamination variable is not one you can measure.
- Reassess if you develop unexplained fatigue or digestive symptoms. Microcystins are hepatotoxins and the early picture is non-specific.
The Honest Summary
Spirulina's real safety question is not pharmacology, it is provenance. The organism does not make toxins; its neighbours in the pond do, and whether they end up in your jar depends on cultivation control and whether anybody assayed the batch. One survey found 44% of products over the limit; a well-monitored production region tested clean across 623 samples. Both are true and they describe different supply chains.
Layered on top of that are three groups for whom spirulina is simply the wrong product regardless of purity: phenylketonuria, autoimmune disease, and anyone who has reacted to it. For everyone else, buy from a producer who tests for the contaminant that actually applies, and treat the absence of a microcystin figure as an answer in itself.
Frequently Asked Questions
Is spirulina contaminated with toxins?
It can be, depending entirely on how it was grown. One analysis of 18 commercial algae supplements found 8 containing cyanotoxins above tolerable daily intake values, while 623 samples from controlled French production stayed within safe regulatory levels for microcystins.
What are microcystins and why do they matter?
They are hepatotoxic compounds produced by certain cyanobacteria that can contaminate open-water spirulina cultivation. Spirulina itself does not produce them, which is why the risk is a production and testing question rather than a property of the algae.
Who should not take spirulina?
People with phenylketonuria, because it is a 60–70% protein product rich in phenylalanine; people with autoimmune conditions or on immunosuppressive medication, because its documented activity is immunostimulatory; and anyone who has had a reaction to it.
Can spirulina cause an allergic reaction?
Yes. A documented case describes anaphylaxis in a 17-year-old on his first ever spirulina tablet, confirmed by skin prick testing against the tablet ingredients. Having never taken it before is not protective.
What should a spirulina certificate of analysis show?
Microcystins, named specifically — microcystin-LR at minimum. Heavy metals and microbiology are useful but test a different contaminant class entirely and say nothing about cyanotoxin content.
Shop Spirulina
Spirulina (120 caps €29) is a single-ingredient product, which removes the blend question but not the provenance one — ask us for the batch documentation before you buy if contamination testing matters to you, as it should. If protein content is the reason you were considering it and phenylketonuria rules it out, that is not a problem another supplement solves. Browse Herbal Botanicals. Free shipping on orders over €50.
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- How to Read a Certificate of Analysis
- Mushrooms and Autoimmune Conditions
- Heavy Metals in Mushroom Supplements
Sources
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- Pinchart PE, Leruste A, Pasqualini V, Mastroleo F. Microcystins and cyanobacterial contaminants in the French small-scale productions of spirulina (Limnospira sp.). Toxins (Basel). 2023;15(6):354. PubMed 37368655
- Le TM, Knulst AC, Röckmann H. Anaphylaxis to spirulina confirmed by skin prick test with ingredients of spirulina tablets. Food Chem Toxicol. 2014;74:309-310. PubMed 25445756
- Giménez-Campillo C, Pastor-Belda M, Campillo N, et al. Determination of cyanotoxins and phycotoxins in seawater and algae-based food supplements using ionic liquids and liquid chromatography with time-of-flight mass spectrometry. Toxins (Basel). 2019;11(10):610. PubMed 31652586
- Lai B, Chen J, Zhang Y, et al. Simultaneous determination of multiclass cyanotoxins in aquatic products, vegetables and algal dietary supplements. Toxins (Basel). 2026;18(3):132. PubMed 41893555