Blue-green algae, also known as Cyanobacteria, are primitive autotrophic prokaryotes. They are a source of food in some parts of the world and patients take the supplemental forms to prevent and treat cancer, viral infections, and for weight loss. Spirulina species are cultured in alkaline fresh water whereas Aphanizomenon flos aquae (AFA) is naturally grown and harvested from Upper Klamath Lake, Oregon, USA. Blue-green algae products frequently contain one or both of these strains of algae.

Small studies have shown that spirulina may be effective against allergic rhinitis ⁽⁵⁾; for managing diabetes ⁽³⁾; for lowering cholesterol in patients with nephrotic syndrome-induced hyperlipidemia ⁽⁴⁾; for relief from chronic pain ⁽²³⁾; may benefit obese and overweight adults by reducing obesity indices ^(24) (28); benefit those with mild cognitive impairment ⁽²⁹⁾; and improve quality of life in ulcerative colitis patients ⁽³⁰⁾. Spirulina may also be effective against chronic hepatitis C virus infection ⁽¹⁹⁾ and adjunctive use, against oral lichen planus ⁽³¹⁾. Foods fortified with spirulina were found to help motor development and prevent morbidity due to upper respiratory infections in infants ⁽²⁵⁾ and positively affect development in children ⁽²⁶⁾.

Supplementation may benefit HIV patients via immunomodulatory effects and by reducing viral load ⁽²⁷⁾. Animal models suggest that spirulina has chemo- and radioprotective effects ⁽⁶⁾, but human studies are lacking. It is important to note that blue-green algae may be contaminated by strains of algae (e.g. Microcystis species) that are toxic.