The differential risk of oral contraceptives: the impact of full exposure history

Third generation oral contraceptives and risk of venous thrombosis: meta-analysis

OBJECTIVE

To evaluate quantitatively articles that compared effects of second and third generation oral contraceptives on risk of venous thrombosis.

DESIGN

Meta-analysis.

STUDIES

Cohort and case-control studies assessing risk of venous thromboembolism among women using oral contraceptives before October 1995.

MAIN OUTCOME MEASURES

Pooled adjusted odds ratios calculated by a general variance based random effects method. When possible, two by two tables were extracted and combined by the Mantel-Haenszel method.

RESULTS

The overall adjusted odds ratio for third versus second generation oral contraceptives was 1.7 (95% confidence interval 1.4 to 2.0; seven studies). Similar risks were found when oral contraceptives containing desogestrel or gestodene were compared with those containing levonorgestrel. Among first time users, the odds ratio for third versus second generation preparations was 3.1 (2.0 to 4.6; four studies). The odds ratio was 2.5 (1.6 to 4.1; five studies) for short term users compared with 2.0 (1.4 to 2.7; five studies) for longer term users. The odds ratio was 1.3 (1.0 to 1.7) in studies funded by the pharmaceutical industry and 2.3 (1.7 to 3.2) in other studies. Differences in age and certainty of diagnosis of venous thrombosis did not affect the results.

CONCLUSIONS

This meta-analysis supports the view that third generation oral contraceptives are associated with an increased risk of venous thrombosis compared with second generation oral contraceptives. The increase cannot be explained by several potential biases.

Occlusive vascular diseases in oral contraceptive users. Epidemiology, pathology and mechanisms

Despite being an unprecedented departure from normal physiology, the combined oral contraceptive is not only highly effective, but it also has a remarkably good safety record. Concerns over safety persist, though, particularly with regard to venous thromboembolism (VTE), stroke and myocardial infarction (MI). Epidemiological studies consistently show an increase in risk of VTE, but the results are more contentious with regard to arterial diseases. Despite 40 years of research, the mechanisms behind these adverse effects are not understood. In this review, we integrate information from published studies of the epidemiology and pathology of the occlusive vascular diseases and their risk factors to identify likely explanations for pathogenesis in oral contraceptive users. Oral contraceptives induce both prothrombotic and fibrinolytic changes in haemostatic factors and an imbalance in haemostasis is likely to be important in oral contraceptive-induced VTE. The complexity of the changes involved and the difficulty of ascribing clinical significance has meant that uncertainty persists. A seriously under-researched area concerns vascular changes in oral contraceptive users. Histologically, endothelial and intimal proliferation have been identified in women exposed to high plasma estrogen concentrations and these lesions are associated with thrombotic occlusion. Other structural changes may result in increased vascular permeability, loss of vascular tone and venous stasis. With regard to arterial disease risk, epidemiological information relating to dose effects and joint effects with other risk factors, and studies of pathology and changes in risk factors, suggests that oral contraceptive use per se does not cause arterial disease. It can, nevertheless, synergise very powerfully with subclinical endothelial damage to promote arterial occlusion. Accordingly, the prothrombotic effects of the oral contraceptive estrogen intervene in a cycle of endothelial damage and repair which would otherwise remain clinically silent or would ultimately progress - in, for example, the presence of cigarette smoking or hypertension - to atherosclerosis. Future work in this area should focus on modification of the effects of established risk factors by oral contraceptive use rather than modification of the supposed risk of oral contraceptive use by established risk factors. Attempts to understand vascular occlusion in oral contraceptive users in terms of the general features of VTE or with reference to atherosclerosis may be limiting, and future work needs to acknowledge that such occlusions may have unique features. Unequivocal identification of the mechanisms involved would contribute considerably to the alleviation of fears over vascular disease and to the development of even safer formulations.

Effect of 1995 pill scare on rates of venous thromboembolism among women taking combined oral contraceptives: analysis of general practice research database

OBJECTIVE

To compare the incidence of venous thromboembolism among women taking combined oral contraceptives before and after the October 1995 pill scare.

DESIGN

Analysis of General Practice Research Database.

SETTING

United Kingdom, January 1993 to December 1998.

SUBJECTS

Women aged 15-49 taking combined oral contraceptives.

MAIN OUTCOME MEASURES

Incidence of venous thromboembolism.

RESULTS

Use of so called "third generation" combined oral contraceptives fell from 53% during January 1993 to October 1995 to 14% during November 1995 to December 1998. There was no significant change in the incidence of venous thromboembolism between the two periods after age was adjusted for (incidence ratio 1.04, 95% confidence interval 0.78 to 1.39).

CONCLUSIONS

The findings are not compatible with the assertion that third generation oral contraceptives are associated with a twofold increase in risk of venous thromboembolism compared with older progestogens.

Cardiovascular events associated with different combined oral contraceptives: a review of current data

Studies of combined oral contraceptive (COC) use and cardiovascular disease have been conducted against a background of low cardiovascular risk in young women, changing COC composition and changing user selection and monitoring. Studies of myocardial infarction have found inconsistent results, possibly because of differences in the prevalence of risk factors (particularly smoking and raised blood pressure) in the populations studied. In the absence of a history of smoking and other conventional risk factors, current users of modern COCs probably do not have an increased risk of myocardial infarction. Neither are former users at risk. Evidence for important differences in the risk of myocardial infarction between formulations is weak and contradictory. Current users of low estrogen dose COCs have a small increased risk of ischaemic stroke although most of the risk occurs in women with other risk factors (notably smoking, hypertension and probably a history of migraine). Former users of COCs do not have an increased risk of ischaemic stroke. There is insufficient information to determine whether major differences in the risk of ischaemic stroke exist between products. Current users appear to have a modestly elevated risk of haemorrhagic stroke, mainly in women older than 35 years; former users do not. Data examining the risk of haemorrhagic stroke in current COC users with other risk factors are very sparse, as are those relating to the haemorrhagic stroke risk associated with particular COCs. Numerous studies have found, with remarkable consistency, an elevated risk of venous thromboembolism among current users of low estrogen dose COCs. The risk is substantially elevated among women with various inherited clotting factor defects. The effects in COC users with other risk factors for venous thrombosis tend to be less pronounced and more inconsistent. A number of studies have found higher relative risks among current users of low estrogen dose COCs containing desogestrel or gestodene, than among users of similar products containing levonorgestrel. A number of explanations, in terms of bias or confounding, have been proposed for these clinically small differences. At best, empirical evidence for these explanations, is weak. The risk of cardiovascular disease of any description is low in COC users. Women can minimise, and possibly eliminate entirely, their arterial risks by not smoking and by having their blood pressure checked before using a COC (in order to avoid its use if raised blood pressure is discovered). Users may decrease their venous thromboembolic risk by their choice of COC preparation although the effects will be modest.

Third- and second-generation oral contraceptives are associated with similar risk estimates for venous thromboembolism

A scientific discussion between 1995 and 1999 addressed the question whether second- and third-generation oral contraceptives (OCs) were associated with different risks of venous thromboembolism (VTE). Results from three epidemiological studies became available in the course of 1995, in which such differences were observed. Although it was unclear at that time whether these observations reflected causality or were induced by bias and/or confounding, some regulatory bodies in Europe restricted the indication for use of third-generation oral contraceptives. Immediate media attention generated a pill scare in those, but also other, countries. Indications for the influence of bias were observed in the initial studies of 1995 and further substantiated in subsequent utilization and prescribing surveys. The most important bias seemed to be related to differences in age and duration of use between third- and second-generation OC users. A number of new studies as well as new analyses in two of the 1995 databases included measures to limit the effect of the identified biases/confounders. These studies observed similar risks of venous thromboembolism with second- and third-generation oral contraceptives. Two other recently published studies did not or could not include the same level of control for confounding and reported similar results as the 1995 studies, thus reconfirming the relevance of the identified confounders. Population data show that the massive switch in the UK from third-generation OCs to second-generation OCs in 1995 has not resulted in a reduction of the incidence of VTE in OC users after 1995, illustrating that the risk of VTE is not determined by the type of low-dose pill used. In addition, data from trend analyses, spontaneous reporting and studies addressing hemostatic mechanisms in pill-users also do not support a potential difference in risk of VTE between users of second- and third-generation OCs.

Multicenter study of the efficacy, cycle control and tolerability of a phasic desogestrel-containing oral contraceptive

OBJECTIVE

To investigate the efficacy, cycle control and tolerability of a phasic oral contraceptive containing ethinylestradiol 35/30/30 microg and desogestrel 50/100/150 microg.

METHODS

A multicenter study was conducted involving 2070 healthy, fertile women, who received study treatment for six treatment cycles.

RESULTS

Most of the participants (79%) had previously been using an alternative oral contraceptive. In 10,408 treatment cycles, two women became pregnant while on treatment (Pearl index, 0.25). The incidence of irregular bleeding was 10% before treatment, rising to 27% at cycle 1, and decreasing to 11% by cycle 6. Irregular bleeding was mainly due to spotting rather than breakthrough bleeding and the incidence of breakthrough bleeding remained below 2.2% for most of the study period. Only 1.8% of women withdrew due to bleeding irregularities. First-time oral contraceptive users initially experienced more irregular bleeding than switchers but these differences lessened over time. The most common adverse events during treatment were headache, breast tenderness and nausea. The incidence of these adverse events fell to below pretreatment levels with continued use.

CONCLUSION

The phasic preparation was effective and well tolerated.