Fundamental differences exist between synthetic hormone-like drugs (conjugated equine oestrogens and medroxyprogesterone acetate) that are not identical in structure to the body's own hormones, and bioidentical hormones (17beta oestradiol and natural progesterone) that are identical in structure to the body's own hormones. Bioidentical hormones are precise duplicates of the hormones produced by the human body. Why does this matter? Well the E3N cohort study which looked at over 80,000 post menopausal French women and found that those taking bioidentical oestrogen and bioidentical progesterone had NO increased risk of breast cancer, but that those taking oestrogen combined with a synthetic progestagen had a 1.69 x increased risk of breast cancer. Synthetic progestogens, in addition to binding to the progesterone receptor, also bind to the glucocorticoid, mineralocorticoid and androgen receptors. This binding can lead to unwanted side effects such as unfavourable glucose metabolism, fluid retention, acne and weight gain. In the central nervous system, synthetic progestogens tend to cause anxiety and irritability whereas bioidentical progesterone is calming.  It is likely that these differences also contributed to the adverse outcomes in WHI, which were contrary to the cardiovascular benefits seen in previous observational trials.
Hormones have so many benefits and hormones prevent us against the diseases of ageing such as cardiovascular disease, osteoporosis and dementia.
The latest British Menopause Society guidelines and the new NICE guidelines in 2015 state:
and in the words of a BMS researcher: "It is imperative that in our ageing population research and development of increasingly sophisticated hormonal preparations should continue to maximise benefits and minimise side effects and risks." 
Here are some interesting papers that highlight some differences between different types of hormone replacement therapy.
 It is imperative that in our ageing population research and development of increasingly sophisticated hormonal preparations should continue to maximize benefits and minimise side effects and risks