Real kefir has no lactose once it is fermented, as that's what the bacteria feed on, the sugar. You should be using the real kefir grains, not the starter. Also the milk is a HUGE variable. If I didn't have access to raw, unpasteurized 100% grass fed milk I would not be making milk kefir at all. You can make water kefir or use raw young coconut water. Drinking kefir will not heal someone with systemic candida overgrowth, and can make you worse! It CAN drastically improve your health once you are stronger and have eliminated the candida foothold.
Raw milk and raw eggs have been used historically by physicians to initiate healing, but the quality of our food sources now is pretty bad, even free range and organic!
Pasteurized, homogenized dairy is a different product altogether, and can only be tolerated by someone with a strong immune system. Factory farmed milk is downright toxic. Kefir can only really help you heal once your candida is beaten back and your immune system is strong.
The store bought kefir is simply milk, sugar, and some guar gum to make it thick - you may as well eat some candy!