New research confirms that the main ingredient in the shots -- depo medroxyprogesterone acetate or DMPA -- can cause significant bone loss when used long term. And, say experts, the younger you are when you begin using this contraceptive, the greater your risk of bone-related problems later in life.

"This is not the first study to observe that use of DMPA may result in bone density loss, [but] our study confirms the results of others," says study author Dr. Abbey Berenson, professor of Obstetrics and Gynecology at University of Texas Medical Branch. What makes this study stand out, adds Berenson, is the design, which was able to detect even subtle differences between the groups of patients who were studied.

From the time we are born, well into middle age, our bone mass is constantly breaking down and re-building, usually in a well-balanced design that keeps bone density -- the thickness and strength of our bones -- in check. In women, one of the key factors involved in the bone-making process is the hormone estrogen.

As we age, however, we start to lose more than we make, causing our skeleton to grow more fragile. In women, the start of this process is often marked by menopause, a time when, not coincidentally, estrogen levels dramatically decline.

Now, researchers say that DMPA, which blocks pregnancy by suppressing estrogen production and stopping ovulation, has the same effect on the bones as menopause -- preventing a woman from making new bone fast enough to keep up with the level of bone loss.

"After menopause, bone is resorbed [lost] at a faster rate than it is made. The mechanism is probably similar in women using DMPA," says Berenson.

The most significant dangers linked to DMPA, say other experts, occur when taken by young women whose skeletons are not yet fully formed.

"A woman's skeleton continues to develop until she is about 25 years of age. To decrease bone mass during this time means maturing with a weaker bone structure, and that can translate into really serious problems by the time she reaches menopause," says Dr. Rogerio Lobo, chairman of Obstetrics and Gynecology at Columbia Presbyterian Medical Center.

Those problems, he says, include a high risk of the bone-thinning disorder osteoporosis, as well as increased risk of broken bones, including hips, knees and ankles.

If you're wondering if all forms of this kind of progesterone therapy have the same risks -- including Norplant, another form of progesterone-only birth control, or even natural progesterone treatments often prescribed for peri-menopausal dysfunctional bleeding disorders -- both Berenson and Lobo say there's nothing to fear.

"An injection of DMPA releases progesterone into the blood stream every day for at least three months [with four injections usually used in a year ]. We do not think that a loss in bone density would be observed with other progesterone-only therapies, in which the duration of use is much shorter, smaller in amount, or not every day," says Berenson.

Lobo agrees: "The levels are much lower, and, particularly in the case of progesterone-only therapy for peri-menopausal symptoms, it's only used for part of the month, mimicking a natural cycle of progesterone."

The new study involved a total of 155 women between the ages of 18 and 33. An injection of DMPA was given to 33 women once every three months for a period of one year. Other women in the study group were given their choice of two different oral contraceptives (both containing estrogen) plus one of two different forms of progesterone: norethindrone (chosen by 28 women) and desogestrel (used by 35). Fifty-nine women who did not use any hormonal contraception served as the control group.

At the start of the study all the women were given bone density tests, measuring the lumbar (lower) spine. Other factors figured into the study included body mass index, calcium intake, exercise and smoking -- all known to have an effect on bone formation and loss.

After 12 months, the bone mass of the women was measured once again.

The result: Overall, the group using DMPA experienced a 2.74 percent rate of bone loss, compared to the pill users who actually gained bone mass during the study time. Women using the norethindrone pill gained 2.33 percent bone mass, while those taking the desogestrel formulation gained 0.33 percent bone mass -- about equal to that of the control group.

"Some women in [the DMPA] group did have an overall increase in their bone density, [but] on average, the DMPA group experienced a loss in their measured bone density," says Berenson.

For Lobo, the question now is whether the bone loss would continue at the same rate, grow worse, or stop completely, if DMPA were continued.

"For the most part, the bone density effects are reversible if you stop taking DMPA. But we don't know the degree to which it would continue with continued use of DMPA for more than a year," says Lobo.

Berenson speculates that bone loss would increase over time with continued use of DMPA, but the degree may also be influenced by other factors including ethnicity, smoking, exercise and nutrition.

What To Do

To learn more about all forms of birth control, visit Planned Parenthood found here.

For information on teens and birth control, click here.

To learn more about preventing osteoporosis, visit the National Osteoporosis Foundation, click here.

Reference Source 101

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