Spirulina is widely sold as a superfood supplement, but it poses real risks for several groups of people. If you have an autoimmune condition, a rare metabolic disorder called PKU, are pregnant or breastfeeding, take blood-thinning medication, or have a history of allergies to algae or similar substances, spirulina may not be safe for you.
People With Autoimmune Conditions
Spirulina stimulates the immune system, which sounds like a benefit until you consider what happens when the immune system is already overactive. In autoimmune diseases, the body’s defenses mistakenly attack healthy tissue. Ramping up that response can trigger new flares or even the onset of a condition that was previously dormant.
A 2025 review published in Springer Nature examined reported cases and found that spirulina ingestion was correlated with the development or flare of dermatomyositis (an inflammatory muscle and skin disease) and pemphigus (a blistering skin condition). The review identified five documented cases, four of them in women between ages 45 and 82. In three cases, spirulina appeared to trigger dermatomyositis; in two, it triggered pemphigus. The authors concluded that spirulina should be used with caution in anyone with an autoimmune disease, with particular concern for these two conditions.
If you have lupus, rheumatoid arthritis, multiple sclerosis, or any other condition where your immune system attacks your own body, the immunostimulatory properties of spirulina could work against you rather than for you.
People With PKU
Phenylketonuria (PKU) is a genetic disorder that prevents the body from breaking down an amino acid called phenylalanine. When phenylalanine builds up, it can cause serious brain damage. People with PKU follow strict diets that limit this amino acid. Because spirulina is extremely protein-dense (40 to 65% protein by dry weight) and contains all essential amino acids, it is a concentrated source of phenylalanine. Harvard Health Publishing specifically notes that spirulina should be avoided by anyone with PKU.
People Taking Blood Thinners
Spirulina can interfere with anticoagulant medications like warfarin. Johns Hopkins Medicine warns that if you take warfarin, you should talk to your doctor before using spirulina because it may affect how well the drug works. Spirulina contains vitamin K, which plays a central role in blood clotting and directly counteracts the mechanism of warfarin-type drugs. Even small, inconsistent amounts of vitamin K from a supplement can make your anticoagulant dose unpredictable.
There is also a separate bleeding concern. A review in the Proceedings of Baylor University Medical Center noted that participants in a clinical trial taking spirulina reported multiple episodes of bleeding, though standard clotting tests did not show significant changes. The review classified spirulina’s bleeding risk as low but still present. This creates a complicated picture: spirulina could either reduce the effectiveness of blood thinners or, in some contexts, contribute to bleeding on its own.
People Scheduled for Surgery
Because of the potential for bleeding complications with supplements, researchers recommend stopping spirulina at least two weeks before any scheduled surgery. This is a general guideline that applies to many dietary supplements, but spirulina’s low-level bleeding risk makes the recommendation especially relevant. If you’re planning a procedure, let your surgeon know about any supplements you take.
Pregnant and Breastfeeding Women
There is no safety data on spirulina during pregnancy or breastfeeding. The NIH’s Drugs and Lactation Database (LactMed) states plainly: no data exist on whether components of spirulina pass into breast milk, and no studies have evaluated its safety or efficacy in nursing mothers or infants.
One documented case illustrates the unknowns. A mother who replaced her iron supplement with a blue-green algae complex containing spirulina, chlorella, and other algae began expressing dark green breast milk on her second day after delivery. Lab analysis found no infection or abnormal cells, and the milk returned to normal color within three days of stopping the supplement. While this particular case was not harmful, it shows that spirulina components do reach breast milk in some form.
The bigger concern is contamination. Products grown in uncontrolled conditions can contain heavy metals, and some can be contaminated with a different species of blue-green algae called Microcystis, which produces liver toxins called microcystins. For a developing fetus or nursing infant, even low-level exposure to these contaminants carries disproportionate risk.
People With Allergy History
Spirulina can cause severe allergic reactions, including anaphylaxis. A literature review published in PubMed Central documented cases where patients experienced generalized itching, burning in the soles of the feet, shortness of breath, and hives after taking spirulina. One patient required emergency treatment with epinephrine, antihistamines, and other interventions.
The patients most at risk tend to have existing allergic conditions. One case involved a man with a history of atopic dermatitis, asthma, and allergic rhinitis triggered by grass, house dust mites, dogs, and cats. Other cases involved patients with oral allergy syndrome and confirmed sensitivity to birch pollen. If you have multiple allergies, particularly to environmental allergens or pollen-related food sensitivities, you may be more susceptible to a spirulina reaction. There is no reliable skin test for spirulina allergy, so the first exposure is essentially the test.
People With Thyroid Disorders
Spirulina contains iodine along with a broad spectrum of other minerals. For most people, this is fine. But iodine directly influences thyroid hormone production, and for someone with hyperthyroidism (an overactive thyroid), additional iodine can worsen the condition. A clinical study on spirulina-containing supplements specifically listed hyperthyroidism as an exclusion criterion, meaning researchers considered it unsafe for those patients to participate. If you have Graves’ disease or another form of hyperthyroidism, or if your thyroid hormone levels are unstable, adding an iodine-containing supplement without medical guidance is risky.
Contamination Risks That Affect Everyone
Even people with no underlying health conditions face a risk that is unique to spirulina and other blue-green algae products: contamination with microcystins. These are natural toxins produced by Microcystis species that can grow alongside spirulina, particularly in open-water or poorly controlled growing environments. The FDA warns that microcystins are invisible, odorless, and tasteless, and at significant levels they can damage the liver and kidneys. In laboratory animals, microcystins have caused tumors.
The supplement industry is not tightly regulated, and spirulina products vary widely in quality. If you do take spirulina, choosing products from manufacturers that test for microcystins and heavy metals, and that grow spirulina in controlled conditions rather than natural lakes, reduces this risk substantially. Third-party testing certifications are the most reliable indicator of a clean product.