Be very careful with the brand of chlorella you take. There is a lot of junk around that is not good to take. Chlorella has to be closely watched and carefully grown, just because of the very fact that it absorbs all the heavy metals in the air and from water, etc. I would not take any brand unless tested and approved by my doctor or a doctor who is totally familiar with the differing qualities of chlorella. I know that I am safe if my own doctor takes it or gives it to his child. This is the case with Biopure. He carefully monitors the quality because literally all patients end up taking chlorella, and if patients end up with unusual reactions, he is certain to hear about it.
Do not forget that one can be allergic to any substance for one reason or another. A friend of mine is allergic to a certain brand of health food store salt!
Some people have problems digesting the cell membrane of chlorella. Cellulase, an enzyme available in health food stores, resolves this problem.
If you take Vitamin C, take it as far away from Chlorella as possible, after meals.
The chlorella should be taken 30 minutes before the meal.
If you have side effects: Most side effects reflect the toxic effect of the mobilized metals which are shuttled through the organism. This is instantly avoided by significantly increasing the chlorella dosage, not by reducing it, which would worsen the problem because --- small chlorella doses mobilize more metals than are bound in the gut; large chlorella doses bind more toxins than are mobilized.
Taking chlorella together with food also helps for some people, even though it is less effective that way. If you have a problem with Chlorella pyrenoidosa, try Chlorella vulgaris - it has a thinner cell wall and is better tolerated by people with digestive problems.
Whatever - make very sure that the chlorella you take is "clean". After all, you are trying to get rid of the toxins and not accumulating more.
Take care.