Spirulina: real benefits, tablets vs powder, and why it isn't a source of B12

What spirulina is

Spirulina is the dried biomass of Arthrospira platensis, a cyanobacterium usually called a blue-green alga. It’s farmed in alkaline fresh water, dried and sold as powder, tablets and capsules.

Some “blue-green algae” products contain AFA (Aphanizomenon flos-aquae) instead of, or as well as, spirulina. AFA is harvested from the wild, from Upper Klamath Lake in the US. Memorial Sloan Kettering and Norway’s NAFKAM both note that contamination is more likely in algae from natural waters than in algae farmed under controlled conditions.

Spirulina benefits: what a serving really gives you

Per 100 g, dried spirulina looks impressive: 57.5 g of protein, 28.5 mg of iron and 6.1 mg of copper, according to USDA data. But nobody eats 100 g. A typical serving is a few grams, so the contribution to your daily needs is modest.

See what 3 g and 5 g provide as a share of the adult reference intake used on UK and EU labels:

What those percentages mean in real life:

• protein – about 1.7 g in 3 g, or 3.5% of the reference intake; at these doses spirulina isn’t a protein source;
• iron – about 0.9 mg in 3 g, roughly 6%; if you’re low in iron, you need a proper iron supplement;
• copper – the biggest share, about 18% in 3 g; count it if your multivitamin also contains copper;
• vitamin B12 – almost no active B12, as explained below.

Spirulina has no authorised health claims on the UK or EU register. So “energy” or “detox” wording on a tub isn’t backed by any regulator’s assessment.

Spirulina and vitamin B12: why vegans can’t rely on it

Spirulina labels often show a high % of the B12 reference intake. Trouble is, it depends how B12 is measured. In a 1999 Japanese study, the standard microbiological test found 6–9 times more B12 in spirulina tablets than a method that only detects forms able to bind the gut’s B12 carrier.

Next, the researchers analysed that “active” part. 83% turned out to be pseudovitamin B12 and only 17% true B12. Humans can’t use pseudovitamin B12. France’s ANSES reaches the same conclusion: spirulina isn’t a reliable B12 source for vegans.

The NHS advises vegans to get B12 from fortified foods or a supplement: fortified breakfast cereals, unsweetened soya drinks or nutritional yeast flakes. We’ll cover forms and doses in our vitamin B12 guide.

Spirulina for vegans: better options

People often buy spirulina to plug gaps in a plant-based diet. According to the NHS, a poorly planned vegan diet can fall short on calcium, iron, vitamin B12, iodine and selenium. Spirulina reliably covers none of them.

What works better:

• B12 – a supplement or fortified foods; this one isn’t optional;
• iron – pulses, wholemeal bread, fortified cereals, dark green leafy veg, nuts and dried fruit; plant iron is absorbed less well, so pair it with vitamin C and avoid tea or coffee with meals;
• calcium – fortified unsweetened soya, pea or oat drinks, calcium-set tofu, sesame and tahini, pulses;
• vitamin D – a daily 10 mcg supplement in autumn and winter, as the NHS advises for everyone.

To check your whole diet, try our vitamin check.

What the research on spirulina shows

The 2023 meta-analysis included 1,076 people. Authors of the 2025 one say the blood pressure effect is small and of uncertain clinical significance. Most trials are small and short, and they used different products. For cholesterol or blood pressure, diet, activity and any medicine your GP prescribes have far better evidence.

Spirulina tablets vs spirulina powder

They’re the same dried spirulina. If the taste makes you wince, go for tablets.

How to take spirulina

• Start low. The most common side effects are stomach upset, diarrhoea and wind.
• Keep to a few grams a day. ANSES sees no clear risk in adults at these doses if the product is clean. At 5 g, by its estimate, spirulina supplies 7–8.5 mg of beta-carotene, which reaches or passes the 7 mg it treats as the daily limit from supplements. US food tables list far less beta-carotene, so the real amount depends on the product.
• Timing doesn’t matter. Take tablets with food or stir powder into a drink.
• Don’t count on B12, iron or protein from spirulina if you’re deficient.
• On regular medicines? Show the tub to your pharmacist. In lab studies spirulina blocked liver enzymes CYP1A2 and CYP2E1, which break down some drugs; whether that matters in people isn’t known.

The main risk: contamination

Cyanobacteria grow in water alongside other species, and some of those make toxins. The best known are microcystins, which damage the liver. MSK lists other possible effects of contamination, including kidney and nerve damage and seizures. Most problems involve wild AFA, but the risk is discussed for spirulina too.

ANSES lists what can turn up in spirulina products:

• cyanotoxins, especially microcystins;
• bacteria;
• heavy metals: lead, mercury and arsenic.

The WHO’s tolerable daily intake for microcystins is 0.04 micrograms per kg of body weight. ANSES advises buying only from suppliers that ensure quality control and traceability and that clearly identify the manufacturer.

Who should avoid spirulina

• Phenylketonuria (PKU). Spirulina contains phenylalanine, and ANSES advises people with PKU not to take it.
• Allergies. ANSES advises against it if you’re prone to allergies; anaphylaxis has been reported.
• Autoimmune conditions. Three patients have been described whose pemphigus flared, or whose dermatomyositis began, after supplements containing spirulina or echinacea. The authors suggest immune-stimulating supplements may worsen autoimmune disease.
• Immunosuppressants. An interaction is theoretical so far, for the same reason.
• Blood thinners and diabetes medicines. Increased bleeding and low blood sugar are theoretical risks.
• Muscle or liver problems. ANSES advises against spirulina if your muscles or liver are vulnerable, because cases of muscle and liver damage have been reported.
• Pregnancy and breastfeeding. There’s no reliable safety data. One newborn developed severe high calcium after the mother took spirulina for a long time.
• Children. They’re more sensitive to microcystins. Only give spirulina to a child after talking to your GP.

Tick what’s true for you:

A rare case of muscle breakdown (rhabdomyolysis) after a month of use has also been reported; it cleared once the supplement was stopped. Dark urine, muscle pain or weakness, or yellowing skin means stopping and seeing a doctor. To check combinations, see our vitamin compatibility chart.

How to choose spirulina on iHerb

1. Decide whether you need it. For protein, iron and B12 there are more reliable sources.
2. Choose spirulina (Spirulina or Arthrospira), not blends with AFA or “wild blue-green algae”.
3. Look for the manufacturer’s batch testing for microcystins and heavy metals.
4. Work out the grams of spirulina per serving and stay within a few grams a day.
5. Ignore the % B12 on the label.

Our label-reading guide explains Supplement Facts panels. More supplements in this group are in energy and heart, and a related green trend is covered in our guide to liquid chlorophyll.