Common side effects of using chlorella include stomach cramping, green stools, flatulence and nausea, according to WebMD. Some people may experience diarrhea after taking chlorella. These side effects may be especially prominent during the first week of taking the supplement.
Chlorella may cause skin to become more sensitive to the sun than usual, notes WebMD. For this reason, it is important to wear sunscreen when taking chlorella. Chlorella may contain allergens that cause breathing problems such as asthma. People who are sensitive to iodine should not take chlorella, as it can contain iodine. People who are allergic to mold should avoid chlorella, as it may cause an allergic reaction.
Because chlorella may cause harmful bacterial growth in the intestines, those with weak intestines may want to avoid chlorella, states WebMD. People with lupus, rheumatoid arthritis, multiple sclerosis or other autoimmune issues should not take chlorella, as it may increase immune system activity and make these conditions worse.
It is not known if chlorella causes problems during pregnancy or breastfeeding, as of 2015, notes WebMD. Because of this, expectant mothers should avoid the supplement unless otherwise directed by a physician. Chlorella may interact with some medications, such as immunosuppressants, so it is important to speak to a doctor before using it alongside other drugs and supplements.