Few side effects have been reported from the ingestion of blue-green algae. However, as blue-green algae can accumulate heavy metals from contaminated water, consuming blue-green algae could increase the body’s load of lead, mercury, and cadmium,² though noncontaminated blue-green algae have been identified.³ Another popular species of blue-green algae, Aphanizomenon flos-aquae, has been found to produce toxins.⁴ A few reports also describe allergic reactions to blue-green algae. Animal studies have found spirulina to be safe during pregnancy.^{5, 6, 7}

There is one case report of a man who developed liver damage while taking spirulina.⁸ As he was also talking three prescription medications, it is not clear whether the spirulina caused or contributed to the liver injury.