Chlorella, also known as sun chlorella or green algae, is a dietary supplement made from single-celled freshwater algae. Supplement manufacturers most commonly use the Chlorella pyrenoidosa or Chlorella vulgaris species. The most common chlorella side effects are diarrhea, nausea, gas, abdominal cramping and green discoloration of the stool. Allergic reactions are possible chlorella side effects for some people.

Proponents claim that chlorella is an an effective treatment for cancer and viral and bacterial infections. They also claim that it boosts immune function, lowers blood pressure and cholesterol levels, treats gastrointestinal disorders and detoxifies the body. There has been very little clinical research regarding chlorella, however. Chlorella side effects are usually mild, but some people experience reactions severe enough to merit discontinuing use.

For users who experience them, chlorella side effects most often present during the first week of use. Approximately one-third of users experience gastrointestinal distress severe enough to discontinue use of chlorella. Photosensitivity, or an unusual sensitivity to sunlight, is another of the possible side effects of chlorella. It may be necessary for someone taking chlorella to wear sunblock during sun exposure, especially if he or she is light-skinned.

It's possible for some people to be allergic to the algae itself, but chlorella supplements can contain iodine, which can also cause reactions. Symptoms of an allergic reaction to chlorella include asthma and difficulty breathing. A reaction to the iodine can cause itching, hives, swelling, tightness of breath, vomiting and diarrhea. If symptoms appear, the person should discontinue its use and seek medical treatment.

Both immunocompromised individuals and those with autoimmune diseases can also experience chlorella side effects. Chlorella use might encourage dangerous bacterial growth in the intestines of immunocompromised users. Those who have autoimmune diseases, on the other hand, might experience a flare in disease activity related to chlorella's purported immune-boosting effects. Chlorella might also reduce the effectiveness of immunosuppressant medications.

Chlorella contains a high concentration of vitamin K, which plays a role in blood clotting. Chlorella might reduce the effectiveness of blood-thinning medications. One should consult a physician before taking chlorella if he or she has been prescribed blood-thinning medications.

Research has shown evidence to suggest that chlorella offers some cancer-fighting benefits. A protein extracted from chlorella has been shown to work prophylactically against the spread of cancer cells in mice. In mice, the same extract has been shown to reduce the side effects of chemotherapy drugs without reducing the drugs' effectiveness.