Spirulina

Spirulina is usually called a blue-green algae, but it is technically a cyanobacterium — Arthrospira platensis or A. maxima — a photosynthetic bacterium that grows in warm alkaline water. That distinction sounds pedantic and turns out to be the most important safety point in this entry, so hold onto it.

By dry weight spirulina is 55 to 70 percent protein, which is extraordinary for a plant-like organism, and it carries a distinctive set of compounds alongside it.

What It Contains

• Phycocyanin, the blue-green pigment that gives spirulina its colour and accounts for much of its antioxidant activity.

• Beta-carotene and other carotenoids, at high concentration.

• Gamma-linolenic acid, an omega-6 fatty acid involved in skin barrier function — unusual in that most dietary omega-6 is linoleic acid, and GLA is a step further along the pathway.

• B vitamins, iron, and chlorophyll.

• Complete protein with all essential amino acids present.

What the Evidence Supports

Spirulina is better studied in dogs than most of the supplement aisle. Published research has examined spirulina supplementation and its effect on immune and gut function in dogs, including antibody response, and reported measurable changes. That is a real canine study rather than an extrapolation from rodents, which puts spirulina ahead of most of what sits beside it on the shelf.

It is still a modest evidence base — a small number of studies, mostly looking at immune markers rather than clinical outcomes. Nobody has shown that a spirulina-supplemented dog gets sick less often. The honest description is a nutrient-dense food with promising immune data and no proven clinical endpoint.

Its other common use is allergic and inflammatory conditions, based on human allergic rhinitis trials and on the GLA and phycocyanin content. Thinner ground, but not absurd.

The Dog and Cat Studies in Detail

Two studies are worth knowing by name, because most spirulina pages cite them loosely.

The immune study. Satyaraj and colleagues (2021) fed 30 adult Husky-cross dogs either a control diet or the same diet with 0.2 percent spirulina for 42 weeks. The spirulina dogs showed:

• Higher rabies-vaccine antibody levels at weeks 9, 15 and 30.

• Higher faecal IgA, an antibody that protects the gut lining.

• A more stable gut microbiome after a bout of exercise stress.

• No rise in C-reactive protein, an inflammation marker.

These are immune markers, not fewer illnesses — which is why we describe the evidence as promising rather than proven.

The tolerance study. A 2023 study in Animals by Ricci and colleagues gave spirulina tablets to 60 healthy dogs and 30 healthy cats for six weeks, raising the amount every two weeks.

• Acceptance: most animals accepted the tablets alone or mixed into food. 52 of 60 dogs and 24 of 30 cats completed the trial.

• Effects: owners reported no significant change in stool, vomiting or scratching. Cats’ coat condition, as scored by owners, improved over the six weeks.

• Side effects: two dogs had diarrhoea, four vomited and one cat developed ear dermatitis. Most cases resolved within three days.

The amounts in that study reached several times the starting amounts in our table below. That tells you spirulina has a reasonable safety margin in healthy animals. It is not a reason to start high — the gradual ramp still avoids most refusals and loose stool.

The Purity Problem You Must Understand

This is the section that matters. Spirulina itself does not produce toxins. But it is a cyanobacterium, and other cyanobacteria produce microcystins — potent liver toxins. If spirulina is harvested from open water or grown in poorly controlled ponds, it can be contaminated with toxin-producing species.

The reason to take this seriously rather than treat it as theoretical: cyanobacterial blooms kill dogs every summer. A dog that swims in or drinks from a pond with a blue-green algae bloom can suffer acute, often fatal liver failure within hours. That is the same family of organisms and the same class of toxin. Blue-green algae poisoning is a genuine seasonal emergency, and it is why "blue-green algae" as a supplement category — particularly wild-harvested products from natural lakes — deserves far more scrutiny than it usually gets.

Cultivated spirulina from a controlled facility with third-party testing for microcystins and heavy metals is a different product from wild-harvested lake algae. Buy the former. If a label will not tell you it has been tested for microcystins, do not give it to your animal.

How Much to Give

Animal · Daily amount

• Cat — A pinch, up to 1/8 teaspoon

• Dog under 15 lbs — 1/8 teaspoon, about 100 mg

• Dog 15 to 35 lbs — 1/4 teaspoon, about 250 mg

• Dog 36 to 65 lbs — 1/2 teaspoon, about 500 mg

• Dog over 65 lbs — 1 teaspoon, about 1,000 mg

• Horse — 1 to 2 ounces, around 30 to 60 grams

Roughly 50 to 100 mg per 10 lbs of body weight is the working guide. Start at a quarter of the figure and build over a week or two — spirulina is strongly flavoured and staining, and a full serving introduced abruptly often produces refusal or loose stool. Judge immune and coat effects over six to eight weeks.

How to Give It

Powder mixes into wet food; it is intensely green and will stain bowls, hands and pale fur around the mouth, which surprises people. Tablets avoid the mess. Give it with food. Because spirulina contains iron, do not give it at the same time as a separate iron supplement without veterinary advice.

Dogs, Cats and Horses

Cats take very small amounts and many dislike the flavour. See cat safety for the wider feline picture.

Horses are a genuinely interesting use. Spirulina is fed in equine practice for allergic airway disease — what used to be called heaves and is now grouped under equine asthma — and for insect bite hypersensitivity, at one to two ounces daily. The mechanism proposed is mast cell stabilisation and reduced inflammatory signalling, which overlaps with what our quercetin entry describes. The equine data is limited and largely clinical impression rather than controlled trial. It is worth saying plainly that for equine asthma the interventions with the strongest evidence are environmental: dust control, soaked or steamed forage, ventilation and turnout. A supplement is an addition to that, not a replacement. Our equine research covers what we have studied, and if your horse competes, check current prohibited substance lists before feeding any supplement.

What to Look For When Buying

• Third-party testing for microcystins. The single most important line on the label.

• Heavy metal testing for lead, arsenic, cadmium and mercury.

• Cultivated, not wild-harvested. Named production facility or country of origin.

• Named species — Arthrospira platensis — rather than a vague "blue-green algae".

• Deep blue-green colour and a sealed opaque container. Phycocyanin degrades with light and heat, and faded, brownish powder has lost activity. Our shelf life entry covers the principle, and it is the same reason we publish certificates of analysis for every batch we make — a supplement you cannot verify is a supplement you are guessing about.

Side Effects and Cautions

• Digestive upset and loose stool, especially on a fast introduction.

• Green staining of stool, fur and everything else. Harmless but startling.

• Autoimmune disease. Spirulina has immune-stimulating activity, which is unhelpful in a condition driven by the immune system attacking the animal's own tissue, and potentially counterproductive in animals on immunosuppressive medication. The same reasoning applies as in our turkey tail mushroom entry.

• Iron content — do not stack with iron supplements unsupervised.

• Contaminated product, which is the one that can cause real harm. Covered above.

• Refusal. Plenty of animals simply will not eat it.

When to Talk to Your Veterinarian

Before starting it in an animal with autoimmune disease or on immunosuppressive drugs, or one already receiving iron. Urgently if your dog has been swimming in or drinking from water with visible surface scum, a paint-like sheen, or an algal bloom, and shows vomiting, diarrhoea, weakness, tremors, seizures or collapse — blue-green algae poisoning progresses fast and is a genuine emergency. Do not wait to see whether it passes.

How It Fits With Hemp

They complement each other on skin and coat without overlapping. Spirulina contributes GLA, carotenoids and B vitamins from the inside; the fatty acid profile of hemp seed oil supports the skin barrier. For a dull coat, running both alongside biotin and fish oil covers several different mechanisms rather than doubling up on one.

One practical note, and it is the recurring theme across this whole batch of entries: both spirulina and hemp are best given with a meal, and the fat in a hemp oil serving genuinely helps absorb the fat-soluble carotenoids in spirulina. Our serving guide has the amounts by weight.

Frequently Asked Questions

Is spirulina safe for dogs? Cultivated spirulina tested for microcystins and heavy metals is safe for most dogs. Untested or wild-harvested blue-green algae is not, because it can carry liver toxins from other cyanobacteria.

How much spirulina should I give my dog? About 50 to 100 mg per 10 lbs of body weight daily — roughly half a teaspoon for a 50 lb dog. Start at a quarter of that and build over a week or two.

Is spirulina the same as the blue-green algae that kills dogs? It is the same broad group of organisms, which is exactly why sourcing matters. Spirulina itself does not make microcystins, but toxin-producing cyanobacteria can contaminate poorly controlled harvests.

Does spirulina help dogs with allergies? The mechanism is plausible and human allergic rhinitis data is suggestive, but there are no canine allergy trials. Treat it as an adjunct, not a plan.

Can horses have spirulina? Yes, one to two ounces daily, and it is used in equine practice for allergic airway disease and insect hypersensitivity. Environmental management remains the higher-evidence intervention.

Which animals should avoid spirulina? Those with autoimmune disease or on immunosuppressive medication, given its immune-stimulating activity, and any animal already on an iron supplement without veterinary input.

For the canine research, see the study on supplementation of diets with spirulina and its influence on immune and gut function in dogs. For the seasonal danger, the Centers for Disease Control publishes guidance on harmful algal blooms and animal illness.

Can I give my dog human spirulina?

Yes, if it is plain Arthrospira platensis with no added herbs, sweeteners or flavourings, and the label or certificate of analysis shows testing for microcystins and heavy metals. Check the milligrams per tablet against the weight table before giving a whole one to a small dog — human tablets are sized for people.

Can cats eat spirulina?

Yes, in small amounts. In a six-week study of 30 healthy cats, most accepted spirulina tablets and owners reported better coat condition. One cat developed ear dermatitis. Start with a pinch and stop if your cat refuses food or vomits.

Is spirulina good for a dog’s immune system?

The one long-term canine study found stronger vaccine antibody responses and higher gut IgA in dogs fed spirulina. That is a measurable immune effect, but no study has yet shown spirulina-fed dogs get sick less often.