Side effects from the supplement spirulina can range from flu-like gastrointestinal discomfort to serious complications of existing health problems. Information from controlled clinical human studies on this supplement is incomplete, however, and spirulina side effects can be uncomfortable at best, and dangerous at worst. Spirulina is a type of blue-green algae sold as a dietary supplement. It usually comes in flakes, powder, or tablets. Due to its extremely high nutritional content, spirulina is often touted as a superfood with the ability not only to nourish the body, but to cure disorders such as cancer, HIV, herpes, and allergies.

The maximum daily dosage of spirulina is 50 grams, though most packages recommend much smaller doses of one to eight grams per day. Spirulina tablets are slow-release, and generally taken once a day. Spirulina flakes and powder can be used as a condiment on food, or added to fruit or vegetable drinks to increase their nutritional content. It can also be made into a paste with water and applied to the face as a cleansing mask.

Even at lower doses, common spirulina side effects might include fever, sweating, thirst, constipation, and gastrointestinal symptoms. The fever and sweating are the result of the body trying to burn up the extra protein in the supplement. Thirst and constipation can be relieved by drinking an extra two pints (1 liter) of water along with the supplement. Stomach upset is caused by the acid in spirulina, but this symptom tends to decrease with time.

More worrying spirulina side effects are caused by blue-green algae harvested from wild sources or poorly controlled environments. Wild spirulina can contain bacteria from animal waste that can cause severe diarrhea or vomiting. Microcystins, a type of toxin produced by algae, can accumulate in the liver and cause liver damage or failure.

Algae absorbs heavy metals from its environment, which might cause headaches, joint pain, gastrointestinal symptoms, and organ damage when ingested. Because of these possible spirulina side effects, blue-green algae health supplements should only be purchased from reputable producers that farm the algae in clean water and test the product and water regularly for toxins and heavy metal buildup.

Other dangerous spirulina side effects can be the result of existing health problems. People with phenylketonuria should not take spirulina because their bodies are not able to process all of the amino acids in the product, and people with autoimmune diseases such as multiple sclerosis or lupus should avoid spirulina because it can stimulate the immune system and worsen the condition.