Estrogen therapy (ET), a form of hormone replacement therapy (HRT), is a useful way of combating the uncomfortable symptoms that often accompany menopause.[1] Roughly 1.5 million women between the ages of 45 and 55 experience menopausal symptoms, which often involve hot flashes, flushing, and night sweats, also known as vasomotor symptoms. Additional symptoms include irritability, mood and sleep disturbances, fatigue, decreased libido, dyspareunia, and atrophic vaginitis. The Women's Health Initiative trial has made the use of estrogen therapy obsolete in terms of preventing vasomotor symptoms, osteoporosis, and cardiovascular diseases. Although estrogen therapy remains a controversial treatment in symptomatic menopausal women, due to the risks potentially outweighing the benefits, there is a definite need for estrogen therapy, particularly in the non-systemic form, in women experiencing distressing symptoms. Without it, a third of women experience at least itching, irritation, dryness, and dyspareunia, leading to decreased quality of life. Localized estrogen treatment often relieves these symptoms and significantly increases the quality of life, which includes life-changing improvements in sexuality, the incidence of urinary tract infections, and incontinence.[2] The method of estrogen delivery is vital in assessing its benefits and uses. For example, the use of estrogen transdermally, in stark contrast to orally, has been linked to a lower risk of deep vein thrombosis, cholecystitis, osteoporosis, and stroke. The use of estrogen therapy has also been beneficial in a patient with an increased risk of osteoporotic fractures with long-term use.

The primary use of estrogen therapy lies in its treatment of menopausal symptoms. Although there is a reduction in the use of estrogen therapy as a preventative treatment, it is still routinely used to treat menopausal symptoms locally. Typically, drugs administered vaginally are used mainly for their local effects, but they can also have systemic effects. Estrogen is a steroid hormone that plays a central role in the reproductive system by altering the transcription of genes in specific organs and tissues, primarily the uterus and vagina. The genes undergo alteration through the act of estrogen on certain receptors, known as nuclear transcription factors. These nuclear transcription factors, once bound by estrogen, are then able to bind to promoter regions in sequences of specific genes and are therefore able to regulate these genes [3]. Once these genes get transcribed, this allows for the development, regulation, and maintenance of the uterus and vagina. One important function of estrogen is its influence on the acid pH in the vaginal canal. The pH of the vaginal lumen in the years preceding menopause typically ranges between 4.5 and 6.0, as it varies relative to the menstrual cycle. Estrogen is responsible for decreasing the pH of the vaginal lumen by acting on the vaginal-ectocervical epithelial cells to increase proton secretion [4]. Once a woman experiences menopause, there is a decrease in estrogen, causing an alkalinization of the vaginal canal to about 6.5-7.0. A pH of above 6.5 correlates with increased risk of vaginal infections, UTIs, dryness, pruritus, and dyspareunia, all of which contribute to the symptoms of menopause. Furthermore, alkalinization of the vaginal lumen has associations with the tendency of forming tumors within the cervix.

The form of administration varies with estrogen therapy and depends on the specific symptoms the treatment is targeting. Essentially, certain symptoms, such as the vasomotor symptoms experienced during menopause, would best be controlled with estrogen therapy that acts systemically. Conversely, symptoms involving the vaginal lumen, such as dyspareunia, vaginal itching, and vaginal dryness, experience more control with locally acting estrogen therapy. Typically, women with an intact uterus receive combination estrogen and progesterone therapy. Women who have had hysterectomies receive estrogen alone. Usually, estrogen therapy divides into two main routes administered either orally or transdermally. Estrogen therapy further subdivides specifically into four common forms of application, which include oral estrogen as in the form of pills, transdermal estrogen, estrogen creams, and estrogen suppositories. Examples of oral estrogen include esterified estrogens, conjugated equine estrogens, ethinyl estradiol, 17 beta-estradiol. The non-oral estrogen therapy is produced in the form of 17 beta-estradiol. Oral and transdermal estrogen are both equally equipped to provide menopausal symptom relief while sparing the bone. While many women would prefer the oral route, it requires a higher dose as its bioavailability is due to the first-pass metabolism in the liver.[5]

There are several adverse effects of estrogen therapy that manifest in different ways depending on the route of administration, and whether that route has local or systemic effects. A study exhibiting the adverse outcomes of hormone replacement therapy showed that unopposed estrogen correlates with an increased risk of endometrial and breast cancer. A large prospective study exhibited that there is a positive relationship between estrogen, used in the form of estradiol, and increased breast cancer risk. This study concluded that the relative risk was 1.7 (95 % CI 1.1 to 2.7) for women who used estrogen for more than nine years and 1.8 (95% CI 0.7 to 4.6) for those who used estradiol for more than six years. Another study showed that there is a correlation between estrogen therapy and the risk of stroke, but that has yet to be confirmed.[6] Hormone replacement therapy, as in estrogen therapy, has also been associated with increased risk of developing venous and pulmonary thromboembolism.[7] Patients taking estrogen therapy for hormone replacement purposes should be thoroughly evaluated and considered on an individual level to assess whether the benefits of estrogen therapy outweigh the risks. Additionally, the most common adverse effects experienced by women taking estrogen alone include breast tenderness, bloating, nausea, headaches, leg cramping, and vaginal or "breakthrough" bleeding.

Hormone replacement therapy, which includes estrogen therapy taken either orally or transdermally, has several contraindications that correlate with the adverse effects and relative toxicity of estrogen-based formulations. These include patients who have a known or suspected history of any form of breast or uterine cancer, a history of deep vein thrombosis or pulmonary embolism, a history of stroke or myocardial infarction, or a history of blood clotting disorders.[7] Those patients who have a history of stroke, particularly ischemic stroke events, might benefit from estrogen therapy as the current literature on the contraindication in stroke patients is controversial.[5] These contraindications do not apply to estrogen taken transvaginally, as in the form of creams or suppositories, as the serum level of estrogen via this route is too low to cause any effect.