Contraceptive injections are long-lasting forms of contraception, usually through the use of depot preparations of progestin medroxyprogesterone acetate (DMPA). They are touted to be highly effective reversible forms of contraception. However, concerns have been raised about their effect on bone density.
DMPA and peak bone mass
Bone mineral density is a term which refers to the mass of minerals deposited per unit volume of bones. It is an indirect measure of bone strength. It is affected by factors such as:
- Body composition
- Exercise especially weight-bearing stress on bones
- Calcium and vitamin D concentrations
- Alcohol ingestion and smoking
- Use of corticosteroids or anticonvulsants
- Altered estrogen levels such as pregnancy, breastfeeding, menopause, and hormonal contraceptives
Peak bone mass refers to the bone mass that has been laid down by the end of the period of growth, or at the onset of adult life. It is the accumulated mass from which later losses occur, such as those that occur during aging, pregnancy, lactation, or immobilizing illnesses. Childbirth-related losses are significant, with up to 8 and 5 percent of density being lost during the period of gestation and lactation, respectively. Recovery, however, is rapid, occurring within 12 months of the event for most women.
Low bone mineral density is a known risk factor for osteoporosis, in which the bones become weaker and more brittle, and are more susceptible to fracture. This risk may increase following menopause because of the loss of estrogen, which enhances bone mineralization.
Bone mineral density increases steeply during adolescence, and much of the adult mineral concentration in bone is gained during this period of life. The first three years immediately preceding the onset of puberty are marked by a rapid increase in bone mineral deposition, which is evident at the femoral neck and the lumbar spine. This four- to six-fold increase rapidly tapers off after menarche, and insignificant deposition of bone mass occurs at two years after this event. The inability to mineralize bone because of a low or inadequate calcium intake, or the presence of factors which inhibit calcium deposition in bone, during adolescence, may have significant effects.
The use of contraceptive injections during adolescence is important in that it leads to a lowering of bone deposition at a time when it is normally at a peak. This reversal of physiological processes may lead to a markedly lower final bone mineral density, which can cause the risk of osteoporosis in later life to shoot up. The loss of calcium is reversible when DMPA is used for less than two years.
Bone pains affect almost a quarter of women using contraceptive injections, seriously enough to discontinue the method. Bone loss was found to occur especially in the lumbar spine and the neck of the femur, which are both sites of osteoporotic fracture in older women. This was not found in women using non-hormonal methods of contraception.
The degree of bone loss depends on the time and duration of use. Thus bone mineral density is reduced by 0.5–3.5% at the hip and spine with one year of use, by 5.7–7.5% after 2 years, and by 5.2–5.4% loss following 5 years of use.
There were no differences with respect to ethnic origin, but younger users (16-24 years of age) suffered greater decreases in bone mineral density than women aged 25 to 33 years. When DMPA is used for five or more years during adolescence, bone density has not been observed to completely return to normal. For this reason, patients who wish to use this method of contraception when they are still very young should try to find out if they have a family history of osteoporosis.
DMPA and fractures
Following cessation of DMPA use, bone density increases, but may not return to normal. It takes longer to recover at the hip, femoral neck, and lumbar spine compared to the rest of the body when the DMPA injection is used for two or more years. Some studies have shown a persistent reduction in bone density in adolescents who have used it for at least two years, but more follow-up is required to establish whether this loss is permanent or not.
The relationship between decreased bone density and fracture risk has been explored most in postmenopausal females. Several workers have shown that DMPA use is associated with a higher risk of bone fractures later, in research conducted in a variety of settings and population groups. The odds may be increased by as high as 1.5 in women who have filled 10 or more prescriptions. However, the presence of confounding factors (such as smoking or previous fractures) in these studies indicates that better-quality research in this area is a priority.
The U.S. Food and Drug Administration (FDA) black box warning against DMPA use was added in 2004 because of the high potential of bone mineral density loss. It states that there is a significant loss of bone density with prolonged DMPA use, which correlates with duration of use, and may not revert to normal even after the drug is discontinued. It also points out the potential for a lower peak bone mass when the use of this injection is started in adolescence or in early adulthood, which may increase the risk for osteoporosis and resulting fractures in later life. Finally, it states that the use of this drug for more than two years is justified only in the absence of other effective and acceptable contraceptive methods.
However, many groups do not agree with the assessment of risk, arguing that accidental pregnancy in adolescents and other women poses a greater danger to health and wellbeing than bone loss. This aspect deserves more serious consideration, since osteoporosis is a disabling condition in later life, when the risk of pregnancy is minimal. There may be other ways to prevent unintended pregnancy without the serious consequences of osteoporosis, including programs which promote abstinence before marriage.
Medroxyprogesterone injections should not be used continuously for over two years if other effective contraceptives are available for use by the patient.
Under 18 years of age, the injection should be used only after a full evaluation of the patient and after considering other reliable and acceptable methods of contraception. Conditions which may make medroxyprogesterone injections unsuitable for contraception include:
- Past history of anorexia
- History of any bone disorder
- History of alcohol abuse or smoking
- Low estrogen levels
- The use of corticosteroids such as dexamethasone, or anticonvulsants such as carbamazepine, some of which contribute to bone loss
- Conditions that increase the risk of falls
- Renal disease, or malabsorption
Periodic regular bone scans such as dual-energy X-ray absorptiometry (DEXA) scans may be performed if risk factors are present, or if the injections are continued for a period longer than this. Regular follow-up with a healthcare provider is mandatory to test for signs of bone loss. It is important to counsel women using DMPA that lifestyle modifications such as regular aerobic and weight-bearing exercise, cessation of smoking, and intake of calcium and vitamin D at age-appropriate levels, do not help to prevent bone loss.
All women, and especially adolescents, who are using or considering prolonged use of this method, should be advised to look into other contraceptive methods so as to avoid this important adverse effect. Properly designed programs should empower women to avert premature sexual involvement so as to reduce the occurrence of unintended pregnancies.