Chlorella side effects: what can you notice?

Short answer: The side effects of chlorella have been studied very little. What has been described comes mainly from individual case reports: allergic reactions, skin reactions in the sun, a blood thinner that worked less well, probably because of the vitamin K in chlorella, and a single reaction in the kidneys. Our packaging does not list a specific side effect, but it does say to consult your doctor first if you are pregnant, breastfeeding, taking medication or have a medical condition.

Last updated: 5 November 2026 · Written by Florian Fermin

If you start using chlorella, you want to know what can go wrong. We sell chlorella tablets, and that is why we list the chlorella side effects one by one here: what is really known, how strong the evidence is and what to do if you notice something. Without scaremongering, and without leaving anything out.

What chlorella is, which species is in our products and whether chlorella is dangerous is covered in our article chlorella: what is it and is it safe.

What side effects can chlorella cause?

There are few large studies that systematically track the side effects of chlorella. What is in the scientific literature is mainly case reports about one or a few people. Such a report shows that something can happen, not how often it happens.

Can you be allergic to chlorella?

Yes. An allergy to chlorella is rare, but it has been described:

• After ingestion: in Australia, a 75-year-old man had a severe allergic reaction (anaphylaxis), with very low blood pressure, wheezing, a rash all over his body and diarrhoea. The doctors saw a clear link in time with swallowing chlorella supplements (Abasszade et al., 2020).
• In children: Swedish research described allergy to green algae, including chlorella, in children (Tiberg et al., 1995).
• Through the air: a pharmacist in Singapore developed asthma symptoms, which the researchers attributed to inhaling chlorella powder at work (Ng et al., 1994). That was about inhaling dust, not swallowing tablets.

If you are allergic to algae, do not use chlorella. Do you get itching, a rash, swelling of the face, lips or throat, or difficulty breathing after chlorella? Stop immediately, and if there is swelling or shortness of breath, call 112.

Can chlorella cause skin reactions in the sun?

This has been described, but a long time ago. In 1984, Japanese dermatologists saw five people who developed swelling and red patches on sun-exposed skin after swallowing chlorella tablets. They identified pheophorbide a as the cause, a breakdown product of chlorophyll that can make the skin more sensitive to light (Jitsukawa et al., 1984).

We have not found any recent reports. If you still get redness, itching or swelling on sun-exposed areas after chlorella, stop and see your GP.

Chlorella and blood thinners: what should you know?

Chlorella contains vitamin K. That matters if you use a blood thinner that works via vitamin K, such as warfarin, acenocoumarol or phenprocoumon. In a Japanese case report, warfarin worked less well after a 75-year-old man started taking chlorella (Ohkawa et al., 1995).

It concerns one person, but the mechanism is known. If you want to start or stop chlorella while using such a medicine, talk to your anticoagulation clinic or pharmacist first.

Can chlorella affect your kidneys?

There is one case report. In a Korean boy who had used chlorella tablets for three months, an acute reaction in the kidneys was found during a school health check. His kidney function improved after he stopped chlorella and received medication. The doctors suspect it was an allergic reaction to chlorella (Yim et al., 2007).

This is also a single case, and it says nothing about how often something like this happens. It is, however, a reason not to give children chlorella without consulting a doctor. If you have a kidney condition, talk to your doctor first.

Does chlorella cause stomach or gut complaints?

Little is known about this. In a small study, 20 adults took 10 grams of chlorella tablets per day plus a liquid chlorella extract for two months; 18 of them completed the study. The researchers found no notable changes in standard blood values (Merchant et al., 2000). It was a small study without a control group, so it says little about rare side effects.

Our packaging does not list a specific side effect. If you do notice stomach or gut complaints after starting, you can:

1. take the tablets during or right after a meal, as the label says;
2. start with fewer tablets and slowly build up to the daily dose;
3. stop if the complaints persist, and talk to your GP.

How many tablets to take per day is explained in chlorella dosage.

Who is chlorella less suitable for?

Talk to your GP or pharmacist first if you are pregnant, breastfeeding, taking medication or have a medical condition. Our packaging says so. Also take extra care if you:

• are allergic to algae;
• use a blood thinner that works via vitamin K;
• are sensitive to sunlight or use medication that makes your skin more sensitive to the sun;
• have PKU: like any protein source, chlorella contains phenylalanine;
• have a thyroid condition. Chlorella is a microalga with little iodine (Verbraucherzentrale Hamburg, 2025), but our label does not state an iodine content. So talk to your doctor about all algae products.

If you take the Spirulina Chlorella Tablets or the combo deal, also read the warnings for spirulina. You will find them in spirulina side effects.

What should you do if you get complaints?

1. Stop taking chlorella if you get complaints you don't trust.
2. Swelling of the face or throat, or short of breath? Call 112 immediately.
3. Persistent or severe complaints? See your GP and take the packaging with you.
4. Let us know. Email the batch number and your complaints to support@getgoyu.com, and we will check the batch. You can request a certificate of analysis (CoA) for our chlorella tablets at the same address.

Summary in 3 points

• The side effects of chlorella have been studied very little. What has been described is mainly allergic reactions, skin reactions in the sun and one reaction in the kidneys, each in individual case reports.
• Chlorella contains vitamin K. If you use warfarin, acenocoumarol or phenprocoumon, talk to your anticoagulation clinic or pharmacist first.
• Do not use chlorella if you have an algae allergy, do not give it to children without consulting a doctor, and talk to a doctor first if you are pregnant, breastfeeding, take medication or have a medical condition.

Frequently asked questions

What are the best-known side effects of chlorella?

Mainly allergic reactions and skin reactions in the sun, known from case reports. How often side effects occur is not known, because there is little large-scale research. Our packaging does not list a specific side effect. If you get complaints, stop and talk to your GP.

Can chlorella cause diarrhoea or stomach pain?

That has not been studied well. If you notice stomach or gut complaints, take the tablets with a meal, start with fewer and stop if it persists. Diarrhoea can also be a sign of an allergic reaction, especially together with a rash or shortness of breath.

Can I use chlorella if I take blood thinners?

Talk to your anticoagulation clinic or pharmacist first, especially with warfarin, acenocoumarol or phenprocoumon. Chlorella contains vitamin K, which can affect how these medicines work.

Is chlorella safe for children?

Our packaging does not give a dosage for children. There is a case report of a reaction in the kidneys in a boy who used chlorella tablets for three months. So do not give children chlorella without consulting a doctor.

Can you use chlorella when you are pregnant?

Our packaging says to consult your doctor first if you are pregnant or breastfeeding. So always talk to your midwife or GP first.

Sources

1. Abasszade JH, Rama Raj P, Tinson AJ. BMJ Case Rep. 2020;13(11):e237054. doi:10.1136/bcr-2020-237054 (case report, 1 person)
2. Tiberg E, Dreborg S, Björkstén B. J Allergy Clin Immunol. 1995;96(2):257-259. doi:10.1016/s0091-6749(95)70016-1
3. Ng TP, Tan WC, Lee YK. Respir Med. 1994;88(7):555-557. doi:10.1016/s0954-6111(05)80344-0 (case report, 1 person)
4. Jitsukawa K, Suizu R, Hidano A. Int J Dermatol. 1984;23(4):263-268. doi:10.1111/j.1365-4362.1984.tb01245.x (case reports, 5 people)
5. Ohkawa S, Yoneda Y, Ohsumi Y, Tabuchi M. Rinsho Shinkeigaku. 1995;35(7):806-807. PubMed 8777808 (case report, 1 person)
6. Yim HE, Yoo KH, Seo WH, Won NH, Hong YS, Lee JW. Pediatr Nephrol. 2007;22(6):887-888. doi:10.1007/s00467-006-0420-z (case report, 1 person)
7. Merchant RE, Carmack CA, Wise CM. Phytother Res. 2000;14(3):167-173. doi:10.1002/(SICI)1099-1573(200005)14:3<167::AID-PTR560>3.0.CO;2-R (pilot study without control group, 20 participants, 2 months)
8. Verbraucherzentrale Hamburg. Algen essen: Auf den Jodgehalt achten, 4 February 2025. vzhh.de
9. Product pages and packaging texts of Chlorella Tablets and Spirulina Chlorella Tablets, consulted 5 November 2026.

You can find all our chlorella tablets on the page buy chlorella.

This article is for information only and is not medical advice. Food supplements are not a substitute for a varied and balanced diet and a healthy lifestyle. If in doubt, pregnant or taking medication, talk to your GP or pharmacist.