Parenteral administration of progestins for hormonal replacement therapy

New options for hormonal replacement therapy (HRT) include non-oral, low-dose sustained delivery of progestins in different formulations. Since the Women's Health Initiative (WHI) study, prescription trends were observed selecting progesterone over other progestins, or delivering the progestin parenterally, in order to reach low systemic levels of the steroid. Such modalities include progesterone vaginal rings or gels, and intra-uterine systems (IUSs). When given via the transdermal and intranasal routes, the hormones are delivered systemically although the first liver impact is by-passed. Of the progestins selected for delivery of low doses by means of an IUS, or transdermally, only very active molecules can be used. An IUS approved for HRT in several countries delivers very low doses of levonorgestrel (LNG) in the uterine cavity. Nestorone is well absorbed transdermally from a gel as well as from a metered dose transdermal system (MTDS) that sprays an invisible film on the skin. Transdermal patches combining oestradiol with either norethisterone acetate or LNG have been available for several years. Although parenteral administration of progesterone or low-dose progestins should in theory be associated with fewer side-effects than oral therapy, the long-term safety of the newer systems warrants further evaluation.