For the most part, Spirulina is safe and free of side effects but not all Spirulina supplements are pure. Spirulina collected from contaminated water and processed in an ineffective manner will be much more dangerous. To avoid major side effects, only take Spirulina supplements that are made with 100% pure and organic Spirulina.

Even pure Spirulina may cause a few minor side effects. These side effects are mild and usually diminish as the consumer builds tolerance to the new supplement. Side effects may include upset stomach, nausea or diarrhea.

People with an autoimmune disease, such as multiple sclerosis, rheumatoid arthritis, or lupus, should avoid Spirulina because it may stimulate the immune system which could potentially make the condition worse.

People with a metabolic condition called phenylketonuria (PKU) should avoid taking Spirulina. People with this rare condition cannot metabolize the amino acid phenylalanine. Spirulina is rich in all amino acids, including phenylalanine.

Women who are pregnant or nursing should always consult a doctor before starting any new supplement.