Chlorella and Spirulina: Risks and Side Effects
Diet & NutritionFood & Ingredients

Chlorella and Spirulina: Risks and Side Effects You Should Know About

Chlorella and Spirulina: Risks and Side Effects You Should Know About
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Updated on March 18, 2026
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Note: The U.S. Food and Drug Administration (FDA) does not approve supplements for safety or effectiveness. Talk to a healthcare professional about whether a supplement is the right fit for your individual health, and about any potential drug interactions or safety concerns.

The rich, emerald hues of supplement powders made from chlorella and spirulina, two types of algae, can make them appear healthy. However, while they’re packed with good-for-you nutrients, like protein and B vitamins, and have potential health benefits such as providing antioxidants, these supplements may also be risky for certain people.

Because of this, it’s important to consult your doctor before starting chlorella, spirulina, or any new supplement, especially if you’re pregnant, have an underlying condition, or take certain medications. And only buy quality products with USP, NSF, or ConsumerLab Approved on the label, which means the product has undergone third-party testing for quality and purity.

Here’s what to know about these sea-greens supplements, and their potential benefits, risks, and side effects.

What Are Chlorella and Spirulina?

Chlorella (Chlorella vulgaris) is a freshwater algae. Many chlorella supplements are rich in nutrients and vitamins that plant-based diets often lack, like vitamin D2, vitamin B12, folate, and iron. Not all chlorella supplements have the same makeup of nutrients, however.

Studies in animals and humans have found that chlorella may benefit the immune system, blood pressure, and cholesterol. This is likely due to the combined effects of its antioxidants and nutrients, though more research is needed.

“It contains all the essential amino acids, which are protein building blocks that our body can’t make,” says Vanessa King, RDN, a spokesperson for the Academy of Nutrition and Dietetics based in Oahu, Hawaii, who specializes in healthy eating and supplements.

Spirulina is a species of blue-green algae plant that grows in oceans and salty lakes, and has been touted for centuries for its high nutritional value. This algae contains iron, beta-carotene, vitamin B1, vitamin B2, vitamin B3, copper, magnesium, and protein.

Spirulina’s nutrients may help aid immune and metabolic function, says John Abroon, MD, a board-certified internist and primary care physician with a private practice in New York City.

Are There Side Effects Linked to Chlorella and Spirulina?

Chlorella and spirulina supplements may have side effects, so it’s important to talk to your doctor before you start taking them.

Possible Side Effects of Chlorella

Chlorella supplements are generally recognized as safe by the U.S. Food and Drug Administration (FDA), and most studies don’t show major side effects. However, some people have reported negative effects such as mild nausea, vomiting, diarrhea, cramping, and gas.

“Chlorella can cause gas or cramping because its cell walls are rich in polysaccharides [complex carbohydrates], which are harder to break down,” Dr. Abroon says. “This can then ferment in the gut and create bloating, especially when someone introduces it too quickly or takes a high-dose supplement.”

There have also been rare reports of kidney irritation called tubular nephritis, which can cause pain and other urination issues, fever, and low back pain.

This is linked to chlorella’s high mineral content, so those with existing kidney issues may struggle to process it, Abroon says.

“Chlorella can also trigger allergic reactions in some people, particularly children,” King adds. This includes asthma and anaphylaxis. Chlorella can also cause photosensitivity, which can make the skin more sensitive to light and susceptible to sunburn.

Possible Side Effects of Spirulina

Spirulina is also generally recognized as safe by the FDA,

 though Abroon says it may cause stomach upset or headaches, particularly when taken in large or concentrated doses (your doctor can recommend the right dosage for you).

These effects are uncommon when consumed in small, food-based amounts such as a teaspoon of spirulina powder mixed into food, Abroon adds.

The Potential Harms of Chlorella and Spirulina

Before you start these supplements, discuss their possible risks with your doctor to determine if you should take them, and for how long.

Both Chlorella and Spirulina May Contain Toxic Metals

“Some chlorella and spirulina supplements have been found to contain trace amounts of heavy metals like aluminum, mercury, nickel, and lead,” King says.

One study from Poland of 52 commercially available spirulina and chlorella supplements reported finding aluminum, manganese, strontium, and zinc in them, though they were within the European Union’s limits for toxic metals. And both organic and nonorganic products were found to be contaminated with small amounts or with residues of substances like caffeine and prescription medications such as metronidazole (Flagyl), carbamazepine (Tegretol), benzocaine, and tramadol (Ultram).

“The main health risks stem from contamination,” Abroon says. “Unregulated products may contain heavy metals, bacteria, or cyanotoxins that could be harmful if taken regularly.”

Contaminants can build up in your body and harm your kidneys, nervous system, and gastrointestinal tract, he adds. To avoid this, look for supplements that are third-party-tested.

Spirulina May Cause Liver Damage

One review suggested the potential of liver damage from spirulina. A Japanese case study linked spirulina use as a possible cause of hepatotoxicity (chemical-driven liver damage) in a 52-year-old man.

 On the other hand, a meta-analysis from Iran of seven randomized controlled trials found that chlorella actually lowered one liver enzyme (aspartate aminotransferase), a marker of liver damage.

“In practical terms, for most healthy adults, the risk of spirulina-related hepatotoxicity appears to be very low — it's just not zero,” Abroon says. Plus, he notes that some findings suggest that certain algae supplements may support liver function (or at least not harm it in healthy people).

Spirulina May Produce Other Harmful Bacteria and Toxins

Repeated exposure to certain toxins in spirulina can strain the liver, limiting its ability to process toxins, says Abroon.

A study out of Greece of five store-bought spirulina products found they contained potentially harmful bacteria and toxins at levels that could cause someone to exceed recommended daily safety limits for microcystin toxin (which is 2400 nanograms for adults weighing about 130 pounds, a conversion based on the World Health Organization’s tolerable daily intake for the liver toxin microcystin-LR). This may be because spirulina is usually grown in open ponds outdoors that can be contaminated by other microorganisms that produce toxins, unlike spirulina itself.

Who Should Avoid Chlorella and Spirulina?

It’s important to speak to your doctor before taking chlorella or spirulina, as they may not be safe for those taking certain medications or with specific conditions.

“People taking warfarin should avoid chlorella, as its vitamin K content can reduce the drug’s blood-thinning effects,” King says.

 “Those with allergies to mold, pollen, or similar environmental triggers may also be sensitive to chlorella.”
Spirulina may also contain the amino acid phenylalanine, so those who have phenylketonuria, a disorder in which your body can’t metabolize phenylalanine, should avoid it.

 Otherwise, phenylalanine can build up and cause brain damage, seizures, or tremors, or harm a developing fetus in pregnant women, among other serious problems.

“Pregnant and lactating women should check with their doctor before using chlorella or spirulina because their detoxification pathways are under extra pressure and any harmful substances could affect the baby,” Abroon says.

The Takeaway

  • Chlorella and spirulina are algae available as supplements and are generally recognized as safe for most healthy people by the FDA, but you should still always speak to your healthcare provider before starting a new supplement — especially if you’re taking medications or have conditions.
  • Chlorella can cause side effects like mild nausea, vomiting, diarrhea, cramping, gas, or kidney irritation.
  • Spirulina’s side effects include mild nausea or headaches, and it may cause liver damage or gastrointestinal problems.
  • These supplements carry the risk of contamination from heavy metals like aluminum, mercury, nickel, and lead, or contamination from other microorganisms, so it’s important to look for a brand that’s been third-party-tested, and to only take the dose recommended by your doctor.
EDITORIAL SOURCES
Everyday Health follows strict sourcing guidelines to ensure the accuracy of its content, outlined in our editorial policy. We use only trustworthy sources, including peer-reviewed studies, board-certified medical experts, patients with lived experience, and information from top institutions.
Resources
  1. Bito T et al. Potential of Chlorella as a Dietary Supplement to Promote Human Health. Nutrients. August 19, 2020.
  2. What Is Spirulina and Why Is It So Good for You? Cleveland Clinic. May 19, 2025.
  3. Barghchi H et al. The Effects of Chlorella Vulgaris on Cardiovascular Risk Factors: A Comprehensive Review on Putative Molecular Mechanisms. Biomedicine & Pharmacotherapy. June 2023.
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  5. Tubulointerstitial Nephritis. Cleveland Clinic. May 28, 2025.
  6. GRAS Notices. U.S. Food and Drug Administration. March 9, 2026.
  7. By the Way, Doctor: Is Spirulina Good for You? Harvard Health Publishing. June 21, 2021.
  8. Sochacka M et al. Spirulina and Chlorella Dietary Supplements — Are They a Source Solely of Valuable Nutrients? International Journal of Molecular Sciences. October 27, 2025.
  9. Sharma R et al. Algal Elixirs: Unraveling the Multifaceted Impact of Spirulina in Human Health. Food Bioscience. December 2024.
  10. Yarmohammadi S et al. Effect of Chlorella Vulgaris on Liver Function Biomarkers: A Systematic Review and Meta-Analysis. Clinical Nutrition Research. January 29, 2021.
  11. Rhoades J et al. Microbiota and Cyanotoxin Content of Retail Spirulina Supplements and Spirulina Supplemented Foods. Microorganisms. April 30, 2023.
  12. Cyanobacterial Toxins: Microcystin-LR in Drinking-Water. World Health Organization. 2003.
  13. Phenylketonuria (PKU). Mayo Clinic. May 13, 2022.
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