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Risk of stroke using oral contraceptives


Since the introduction of oral contraceptives there have been concerns about their safety. Many papers have examined the association between ischaemic stroke and oral contraceptive use. This meta-analysis pools the results from some of these studies.

Message

The estimated risk of ischaemic stroke for non-smoking, normotensive women using low oestrogen oral contraceptives is very small: approximately one in 24,000 per year. To put this into perspective, the risk of being killed on the roads in the UK is higher at about 1 in 17,000, and the UK has some of the safest roads in the world.

Reference

LA Gillum et al. Ischaemic stroke risk with oral contraceptives. Journal of the American Medical Association 2000 284: 72-78.

Search

The literature was searched from 1960 to 1999 using Index Medicus (before 1966), MEDLINE (after 1966), BIOSIS (after 1985) and Dissertation Abstracts Online. All languages and publication types were included. Bibliographies of identified papers were then reviewed. Textbooks and an expert were also consulted. Two investigators independently applied the following inclusion criteria:

Sixteen studies met the inclusion criteria (three cohort and thirteen case-control). The studies were small; case numbers in the case-control studies ranged from 19 to 697. Oestrogen doses varied (less than 50 µg, 50 µg and more than 50 µg). Risk estimates were not consistently calculated for current versus non-current use (e.g. some studies calculated current use versus never used). The definition of current use varied with last use ranging from two weeks to 12 months before the stroke.

Results

Stroke risk was associated with current oral contraceptive use but not with past use.

The risk of stroke varied with the characteristics of studies entered into the analysis.

When all the studies were included in the analysis, oral contraceptive users had over two and a half times the risk of ischaemic stroke compared with those not currently using oral contraceptives.

The risk of stroke was smaller in studies which had adjusted for smoking and hypertension, used population controls rather than hospital controls and examined smaller oestrogen doses (see Table 1).

Table 1. Influence of study characteristics on estimation of relative risk of ischaemic stroke among oral contraceptive users.

Study Characteristics

No. of Studies

Relative risk (95% confidence interval)

All studies

16

2.75 (2.24 - 3.38)

Adjusted for smoking and hypertension; examined low oestrogen dose (less than 50 µg)

6

2.04 (1.51 - 2.76)

Used population controls; adjusted for smoking and hypertension; examined low oestrogen dose (less than 50 µg)

5

1.93 (1.35 - 2.74)

 

Comment

This paper provides a lovely example of how results can be misleading. Firstly, the differences in study design (e.g. many early studies did not adjust for smoking or hypertension) made the estimated risk of stroke much higher than it really is. Secondly, the estimated relative risk of stroke is double (based on a handful of studies with few case numbers, 'estimated' is the key word here), but the absolute risk is one additional ischaemic stroke per year per 24,000 non-smoking, normotensive women using low-oestrogen oral contraceptives. In other words, the risk is very small and similar to risks we usually accept as part of living. The information makes it possible for women and their partners to decide if the risk is acceptable to them.