What Causes White Vaginal Discharge?

hi a friend of mine discuse of having a white discharge from her vagina for a long time(for some years) and she dosen t no what it is then,Recently I took her to hospital the doctor confirm she s having a yeast infection,they prescribe some drug and vaginal cream for her after taking the treatment she did not see any changings,pls what can she use to treat this infection we your help