Spirulina is generally considered to be safe and well-tolerated. The most commonly reported side effects include gastrointestinal upset (e.g., abdominal cramps, diarrhea, nausea, vomiting), fatigue, dizziness, headache, and skin irritation (e.g., itching, rash).[1]
There have been several case reports of more severe side effects with spirulina supplementation. These include rhabdomyolysis (a severe type of muscle breakdown),[2] anaphylaxis (a life-threatening allergic reaction),[3][4] liver toxicity,[5] and the initiation or exacerbation of autoimmune skin conditions.[6][7]
Although this has not been thoroughly investigated, spirulina may inhibit some cytochrome P450 enzymes, namely CYP1A2 and CYP2E1, which could lead to drug interactions.[8]
Manufacturing practices and growth environments can influence the quality and safety of spirulina supplements. First, spirulina can accumulate heavy metals from its environment, raising the concern of contamination.[9] One study that checked 25 brands of spirulina for mercury, nickel, and platinum found that none contained these heavy metals beyond acceptable levels,[10] but choosing a spirulina supplement that is tested for heavy metals should still be a consideration. Second, contamination with other species of cyanobacteria could introduce potentially toxic compounds like microcystins.[11]