I'm taking high strength calcium and vitamin D3. But only because I now have bone thinning. Not as a result of cancer or chemo, but because I have RA and both mum and gran had osteoporosis. But I am now also taking Anastrazole, which would put my bones at an even greater risk.
I have used calcium and vitamin D for a number of years (normal strength) and I still got breast cancer. My opinion is, if you eat a healthy diet, keep yourself at a reasonable weight, get some regular exercise and eat plenty of fruit and vegetables, don't drink too heavily and don't smoke, you MIGHT reduce your risk of breast cancer, but nothing is guaranteed. I've read countless studies, one will say this will reduce your risk, another says no, but something else will reduce your risk. Just eating and drinking sensibly is all we can do. A good healthy diet should give the average person enough calcium, and all the necessary vitamins our bodies need, without using supplements. Only certain people need supplements. I take Omega 3 fish oils (for my RA) and the aforementioned high strength calcium and Vit D3.
Has anyone heard of taking Vitamin D and calcium? There's no evidence but a few studies suggested it could prevent the risk of breast cancer, decrease density and other health issues. It you have a low intake of Vitamin D you could have a risk of breast cancer.