Research in the American Journal of Obstetrics and Gynaecology in 2002 shows that increased oestrogen levels in the blood is associated with increased growth of Candida in the vagina, and that growth of Candida appears to be stimulated by oestrogen in a study publicised in Clinical Infectious Diseases in 2002. A study published in the American Society of Microbiology in 2006 found, if certain types of oestradiol (a specific type of oestrogen found in birth control pills) are added to candida cells, this oestrogen has been shown to increase the number of germ tube and length – developments that support candida overgrowth. After starting the birth control pill, many women complain of developing chronic thrush and bloating and flatulence due to the candida overgrowth in their gut.

Oestrogen promotes production of glycogen (glucose) within the vaginal cells, and increased glycogen acts as an ideal source of food for growing Candida. Oestrogen also acts on Candida yeasts and promotes its growth, and improves its ability to cling onto vaginal epithelial cells as shown by this study in PLOS Pathogens published in 2014.

These changes together can encourage the overgrowth of Candida, and further studies are being conducted to gain a better understanding of these mechanisms.

This explains why pregnant women, and those who are on the combined oral contraceptive pill (COCP) or hormone replacement therapy (HRT) are more at risk of developing thrush, as all these situations can be associated with potentially high oestrogen levels.