It's a reasonable concern, but all studies are flawed in some way. From the perspective of clinical practice, this is how most clinics assess supplement use - b

It's a reasonable concern, but all studies are flawed in some way. From the perspective of clinical practice, this is how most clinics assess supplement use - by asking the patient. In the absence of any better data (and it surely could be better) it at least offers some guidance when patients ask if they should or shouldn't take supplements during chemotherapy. I would tell them that there's limited evidence, but it points towards avoiding supplements.