Progestogens alone have a very limited effect on bone mineral density (BMD). But, pretreatment with estrogen for 4–7 days has been shown to induce progesterone receptors in osteoblasts [9]. This explains the beneficial effect on BMD, in all clinical trials in which progestogens were added to estrogen.

Medroxyprogesterone acetate (MPA) decreases BMD, both in the hips and spine, in the first 2 years of use, followed by a slight increase [10]. The effect of norethisterone acetate (NETA) with a low dose of 17β-estardiol on BMD has been investigated in a randomized placebo control study [11]. There was a significant increase in BMD both in the lumbar spine (5.2 %) and the hip (3.1 %) compared to the placebo group (−0.9%). Serum concentration of osteocalcin decreased by approximately 34 %, bone-specific alkaline phosphatase decreased by about 30 %, and C-terminal propepetide of type I collagen decreased by 20 %. Norethisterone is a synthetic progestin derived from 19-nortestosterone. It binds and activates the progesterone receptor twice as much as progesterone itself, with low androgenic and estrogenic activities attributed to its metabolites [12]. NETA alone has been studied in male castrated mice. In this study NETA alone was found to have a slightly protective effect against bone loss [13]. Controversially, inhibition of the nuclear progesterone receptor has been found to augment bone mass, resulting in higher BMD [14].

Nightly micronized progesterone (300 mg) has been shown to cause an overall decrease in the number of daily hot flushes and night sweats by 59 % [15]. Extension of this study has shown that micronized progesterone is also effective for severe vasomotor symptoms and that progesterone withdrawal is not followed by a rebound increase in vasomotor symptoms [16]. A similar effect has been shown for medroxyprogesterone acetate (MPA) 10 mg/day. Prior et al. [17], carried out a randomized double-blind trial in women after overiectomy. Patients received either conjugated equine estrogen (CEE), (0.6 mg/day) or MPA for 1 year. MPA was found to be equivalent to CEE in the control of vasomotor symptoms in women treated immediately following the surgery [17]. Oral progestins have been shown to be effective for vasomotor symptoms in several randomized placebo controlled trial [18, 19].

Estrogen alleviates hot flushes by lowering levels of serotonine and noradrenaline in the brain [19]. Progestogens may act in a different manner. Progesterone acts on the hypothalamus changing the frequency of LH pulses, increasing basal temperature and stimulating respiration. Therefore progesterone and progestins have various effects on the hypothalamus, from which hot flushes are thought to generate [20].

Progestins, when given alone (e.g. progestin only contraceptive pills), carry little, if any, risk of VTE. Randomized controlled studies and meta-analyses of observational studies suggest that the risk of venous thrombo-embolism (VTE) is higher among users of combined estrogen and progestogen than among users of estrogen alone. Oral estrogens increase the VTE risk while transdermal estrogens appear to be safe with respect to thrombotic risk [21]. However, MPA, by activating glucocorticoid receptors, potentiate the vascular effects of thrombin [22]. The ESTHER study (Estrogen and Thromboembolism Risk) looked into the risk of VTE in French postmenopausal women treated with HRT. This study was the first to establish a differential association of VTE risk related to the progestogen used. The results were irrespective of the route of estrogen administration. The results showed that micronized progesterone and pregnane derivates are safer with regard to VTE risk. The norpregnane derivatives were associated with a significant increase in VTE risk [23].

The norpregnanes are potent progestogens with antiestrogenic activity. Women suffering from hyperestrogenic effects, such as breast tenderness or endometrial hyperplasia are more likely to benefit from this type of progestogens.

Progesterone is metabolized in the brain by 5α-reductase to 5α-dihydroprogesterone. This in turn is further metabolized by 3α-hydroxysteroid dehydrogenase to the neurosteroid allopregnanolone.

The neurosteroids are modulatory ligands for a variety of neurotransmitters and nuclear steroid hormone receptors. Allopregnanolone crosses the blood-brain barrier. It has been shown in rodents, that allopregnanolone is an efficacious proliferative agent (both in vitro and in-vivo studies) [24]. It also decreases amyloid protein in human neural stem cells [25]. Progesterone metabolites exert considerable sedative effects after binding to the GABAA receptor [26]. Neurosteroids such as pregnanolone affect synaptic functions and myelinization. Their action is mediated through inhibition of the glycogen synthase kinase (GSK-3β) pathway (much like most bipolar mood stabilizers such as lithium) [27]. Neurosteroids are modulatory ligands for a variety of neurotransmitters and nuclear steroid hormone receptors. In a mice model with experimental autoimmune encephalomyelitis it has been shown that both the spinal cord and the brain are sensitive to the protective effects of progesterone. Progesterone has been shown to reduce inflammatory reactions commonly seen in MS, by the direct effect of progesterone on astrocytes and microglia [28].