The process of taking
hormones of the opposite physical
sex for a
transsexual to appear more like that of the other gender, mainly by development of the
secondary genderic signs identified with that gender. The amount of effect depends on the person's
genetics and
age - the younger someone is when starting on the hormone therapy, the
stronger the effect of the hormones.
Taking estrogen causes breast growth, softer skin, and makes a person more emotional and sensitive.
Taking testosterone causes facial hair growth, a deeper voice, and may make one more aggressive and competitive, though there is debate about the last part.
Hormone therapy in the United States is usually done in a manner closely resembling the HBIGDA Standards of Care for pre-operative transsexuals. Usually a person is required to be in a program of psychotherapy for a minimum of three months, with a letter of approval from the therapist for starting hormones.
The method of administering the hormones varies from doctor to doctor. Some rely solely on injections, others use orally administered hormones in pill form, and some combine the two.
Hormone therapy is not a temporary thing. Because the body of a transsexual will never have proper hormone levels for their chosen gender, the person must continue their hormone regimen for the rest of their life. After Sex Reassignment Surgery, the dosage can usually be lowered, and the drugs for blocking the undesired hormones can be discontinued, but at least something will continue to need to be administered regularly.
Hormones should never be taken without a doctor's care. They have serious effects, and if not properly monitored, can cause serious physical harm and have significant effects on the mental state. Also, taking too much of a hormone, because of how the body processes the hormones, can actually result in decreasing the effects of that hormone.
In the past, doctors used to cycle the hormone dosage for male-to-female transsexuals, to attempt to give them the monthly cycle that occurs naturally in the bodies of genetic women. However, this practice has ceased, as there is no real need to create the cycle. Interestingly, even with consistent dosage of hormones throughout the month, some transsexual women do still seem to have a monthly cycle. On a schedule as regular as a genetic women, there may be a period of moodiness that can be quite noticable. Some even have "cramps" occasionally. How? While the reproductive organs are not present, the muscles that are involved are present (though aligned slighly differently), and as the body adjusts to the estrogen, the body may start triggering them at that point.
(I can vouch for all this last part - I do have a monthly cycle that is identifiable by my other half, and the past few months I've had some unusual muscle tightness at that time that I've never experienced before, but appears to be of the same type.)