Spirulina supplement: what it may do and how to buy a clean one
We research products from makers' specs and health sources; we do not run hands-on tests. Links to Amazon may earn us a commission at no cost to you. How this works.
- Organic spirulina tablets. Best for: Adults who want a simple, measured daily amount. See options on Amazon
- Organic spirulina powder. Best for: People who want gram-level amounts in smoothies. See options on Amazon
- Spirulina capsules. Best for: People who prefer capsules to tablets or powder. See options on Amazon
What spirulina is
NIH's LiverTox database explains that spirulina is made from blue-green algae (cyanobacteria), mainly Arthrospira platensis and maxima. It has been used as food for centuries in Mexico and Central America and is now sold as a dietary supplement.
LiverTox says spirulina is particularly rich in protein (65% to 70%) and contains vitamins, beta-carotene, polyunsaturated fatty acids, calcium and iron. Its blue-green pigment, phycocyanin, makes up as much as 20% of its dry weight and has been proposed as its main active ingredient.
What the research shows
| Review | What it found |
|---|---|
| 2021 meta-analysis, 5 trials, 230 people | Lower systolic and diastolic blood pressure, especially with hypertension |
| 2025 meta-analysis, 17 trials | Small drops in body weight, BMI and body fat; no change in waist size |
| LiverTox overview | Limited human studies; not proven to treat any condition |
Blood pressure. A 2021 meta-analysis in Nutrients pooled 5 randomized trials with 230 people taking 1 to 8 grams a day for 2 to 12 weeks. Systolic pressure fell about 4.6 mmHg and diastolic about 7 mmHg on average, mostly in people with high blood pressure. The authors called for more precise trials.
Body weight. A 2025 meta-analysis of 17 trials found spirulina reduced body weight by about 1 kg, BMI by 0.4 and body fat by under 1 percentage point, with no significant change in waist circumference. Effects were larger with higher doses and longer use.
Other claims. LiverTox lists uses such as ADHD, allergies, diabetes, stress, fatigue, anxiety and depression, but says spirulina has not been proven effective for any medical condition.
Purity and label accuracy
LiverTox notes spirulina grows in warm, alkaline open lakes and can be contaminated with other blue-green algae that produce toxins called microcystins. Contamination is one possible explanation for rare reports of liver injury.
A 2022 study in Foods tested spirulina supplements sold in Slovenia and found 86.7% of labels had inappropriate declarations of mineral content, and that added ingredients lowered quality in some products. It also found iron content was often high on paper but mostly in a less absorbable form. The authors called for stricter quality control.
- Choose brands that state third-party testing for microcystins and heavy metals.
- Prefer pure spirulina over blends with fillers.
- Do not count on spirulina as a reliable iron source.
Safety
- Generally well tolerated. LiverTox says studies found spirulina safe without significant adverse events, and animal studies showed no toxicity even at high doses.
- Liver. Liver injury is very rare, if it occurs at all; the one published case recovered fully after stopping, and the person was also taking other drugs that can affect the liver.
- Medicines and conditions. NOW Foods advises consulting a physician if pregnant or nursing, taking medication or managing a medical condition.
- Adults. NOW Foods labels its tablets for adults only.
Tablets, powder or capsules?
- Tablets. NOW Foods' Organic Spirulina gives 1 g (1,000 mg) of Arthrospira platensis per 2 tablets daily, with no other ingredients.
- Powder. Easy to add to smoothies or food when you want larger amounts.
- Capsules. Convenient, but you need more of them for gram-level amounts like those used in studies.
- Spirulina and chlorella combos. Chlorella is a different alga; see our chlorella guide.
How to choose spirulina
- Species named. Look for Arthrospira platensis.
- Organic and tested. Prefer certified organic with testing for microcystins and heavy metals.
- Pure product. The Foods study found additives lowered quality in some products.
- Realistic expectations. Effects on blood pressure and weight in studies were modest; it is not a substitute for treatment.
What to look for when you buy
Organic spirulina tablets
Best for: Adults who want a simple, measured daily amount
NOW Foods lists 1,000 mg of organic Arthrospira platensis per 2 tablets with no other ingredients.
Watch out: Ask your provider if pregnant, nursing or taking medicine. Check for purity testing.
See options on AmazonOrganic spirulina powder
Best for: People who want gram-level amounts in smoothies
Studies used 1 to 8 grams a day, which is easier to reach with powder.
Watch out: Measure servings carefully. Choose brands that state contaminant testing.
See options on AmazonSpirulina capsules
Best for: People who prefer capsules to tablets or powder
Capsules are an easy alternative to tablets and powder.
Watch out: Smaller amount per capsule. Avoid blends with fillers.
See options on AmazonQuestions people ask
- What is spirulina good for?
- It is a protein-rich food supplement. Meta-analyses suggest modest drops in blood pressure and body weight, but it has not been proven to treat any condition.
- Is spirulina safe?
- Generally yes. Liver injury is very rare. The main concern is contamination with toxin-producing algae, so buy tested products.
- How much spirulina should I take?
- Follow the label. NOW Foods directs 2 tablets (1 g) daily; studies used 1 to 8 grams a day.
- Is spirulina a good iron source?
- Labels may show high iron, but a 2022 study found it is mostly in a less absorbable form.
- Does spirulina help weight loss?
- A 2025 meta-analysis of 17 trials found a small average drop of about 1 kg.
Sources
- Spirulina - LiverTox (ncbi.nlm.nih.gov), National Institute of Diabetes and Digestive and Kidney Diseases
- Effect of Spirulina Supplementation on Systolic and Diastolic Blood Pressure: Systematic Review and Meta-Analysis of Randomized Controlled Trials (pmc.ncbi.nlm.nih.gov), Nutrients (PMC8468496)
- Effects of spirulina supplementation on body composition in adults: a GRADE-assessed and dose-response meta-analysis of RCTs (pmc.ncbi.nlm.nih.gov), Nutrition and Metabolism (PMC12175388)
- Nutritional Quality and Safety of the Spirulina Dietary Supplements Sold on the Slovenian Market (pmc.ncbi.nlm.nih.gov), Foods (PMC8954120)
- Spirulina Organic 500 mg Tablets (www.nowfoods.com/products/supplements/spirulina-organic-500-mg-tablets), NOW Foods