Spirulina is a popular supplement and ingredient made from blue-green algae.

Although it has several benefits, you may wonder if it has any side effects.

This article reviews the potential downsides and side effects of spirulina.

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Spirulina is a type of blue-green algae that grows in both fresh- and saltwater. It’s also commercially produced for use in food and supplements (1, 2).

Because it packs 60% protein by weight, as well as various vitamins and minerals, it’s widely used as a food source in certain parts of Mexico and Africa (1).

What’s more, it’s a good source of healthy polyunsaturated fats and the antioxidants C-phycocyanin and beta carotene (1, 3).

As a supplement, it’s touted for its antibacterial, antioxidant, anti-inflammatory, immune-boosting, and cholesterol-lowering potential (4).

Summary Spirulina is a blue-green algae commonly used as a dietary supplement. It may provide antioxidant, anti-inflammatory, and immune-boosting benefits.

Although spirulina is generally considered safe, it may have some side effects and drawbacks — especially for people with certain health conditions (2, 4).

Here are some of the potential side effects and downsides of spirulina.

May be contaminated with toxins

Spirulina harvested in the wild poses a significant risk of contamination. The algae may harbor toxins if it grows in a body of water that’s polluted with heavy metals, bacteria, or harmful particles called microcystins (2).

In fact, microcystins are produced by blue-green algae as a defense mechanism against predators. When consumed in high amounts, they’re toxic to your liver (5).

Microcystin-contaminated algae supplements have been found in Italy, North America, and China, and these compounds are a growing public health concern due to their liver effects (6, 7, 8).

Spirulina grown in controlled environments is lower in microcystins, as scientists have developed methods to remove this compound, as well as limit its production (8, 9).

May worsen autoimmune conditions

Because spirulina boosts your immune system, it may worsen certain autoimmune diseases — such as lupus, multiple sclerosis, and rheumatoid arthritis — in which your immune system attacks your body (2).

Spirulina bolsters your immune system by strengthening immune cells called natural killer (NK) cells, which attack perceived threats on a cellular level (10).

Animal and human studies show that this effect may help slow tumor growth, improve resistance to illness, and decrease inflammation (10, 11, 12, 13).

However, by strengthening the NK cells in people with autoimmune conditions, this algae may exacerbate these conditions.

Spirulina supplements have also been linked to severe autoimmune responses affecting your skin and muscles, although this side effect appears to be very rare (14, 15).

If you have an autoimmune condition, you should avoid spirulina and other algae supplements (2).

May slow blood clotting

Spirulina has an anticoagulant effect, meaning that it can thin your blood and increase the length of time it takes for blood to clot (2, 16).

Clotting helps prevent excessive bleeding or bruising when you’re injured (17).

For those taking blood thinners or who have bleeding disorders, spirulina may be dangerous because it could lessen your blood’s ability to clot, causing more bruising and bleeding (2).

While some studies suggest that spirulina does not affect blood clotting time, little is known about its effects on people who are already taking blood thinners (18, 19).

Thus, you should avoid spirulina if you have a bleeding disorder or are on blood thinners.

Other downsides

Some people may be allergic to spirulina. In severe cases, reactions can be fatal (20).

According to one study, people with other allergies are more likely to react negatively to spirulina than those without other allergies. To be safe, those with allergies should avoid this supplement or consult their healthcare provider before using it (21).

Spirulina and other algae also contain phenylalanine, a compound that people with phenylketonuria (PKU) — a rare inherited condition — should strictly avoid (2).

Some of the minor side effects of spirulina may include nausea, insomnia, and headaches. Still, this supplement is widely considered safe, and most people experience no side effects (2).

Summary Spirulina may be contaminated with harmful compounds, thin your blood, and worsen autoimmune conditions. Some people may be allergic, and those with PKU should avoid it.

As spirulina may have some drawbacks, especially in certain populations, it’s best to consult your healthcare provider before taking it.

To avoid spirulina that has been contaminated with microcystins or toxins, only purchase products from trusted brands that have been tested by third-party organizations, such as U.S. Pharmacopeia (USP), ConsumerLab, or NSF International.

Keep in mind that even certified products may not be completely free of contaminants, as dietary supplements are largely unregulated in the United States.

Summary Purchasing from trusted brands can reduce your risk of contamination. However, there’s no guarantee that spirulina products are 100% contaminant-free.

Although widely considered safe, spirulina has several potential side effects.

Some supplements may be contaminated by toxins. What’s more, this algae may worsen some autoimmune conditions and thin your blood.

You should avoid spirulina if you take blood thinners or have an autoimmune condition, bleeding disorder, allergies, or PKU.

If you’re unsure whether this supplement is right for you, consult your medical practitioner.