From your cancer team. The nurses dealing with you might put you in touch with a nutritional advisor. Some diets, some supplements etc can interfere with the drugs you're taking, so always direct these things at the people treating you. It might be a case of there are no restrictions that you can or can't do, but yeah, let those treating you tell you that. The ability of your immune system also plays an important part. Things you can normally take because your immune system keeps things under control might not be the case should it take a severe dip, which it can.
I know my wife was advised to stay away from things containing live bacteria/pro biotics at some stages throughout her treatment.
I make my own kefir and took it throughout my IV chemotherapy. I'm now on oral chemotherapy for life as I'm stage 4 and I continue to take it regularly.
I checked beforehand and the problematics seem to be in the form of supplements not live cultures. For me anyway. It's worth asking your oncologist as treatments can be very different.
