Table 2

Association between use of hormonal contraceptives and severe asthma exacerbation rates in reproductive-age women and by body mass index and smoking*

Hormonal contraceptive (HC) useStratified analyses by BMIStratified analyses by smoking status
Incidence rate ratio (95% CI)†‡Hazard ratio (95% CI)†§
<25 kg/m2 25–29.9 kg/m2 ≥30 kg/m2 Non-smokersEver-smokers
Any HC during any previous or current year
 None11111
 Yes0.92 (0.88 to 0.96)0.93 (0.88 to 0.98)0.96 (0.92 to 1.01)0.92 (0.88 to 0.96)0.96 (0.93 to 1.00)
Any HC during current year of asthma exacerbation
 None1111
 Yes0.96 (0.93 to 0.99)0.98 (0.95 to 1.02)0.94 (0.91 to 0.96)0.99 (0.96 to 1.02)0.94 (0.92 to 0.96)
Type of HC during any previous or current year
 None11111
 Progestogen-only0.91 (0.79 to 1.04)0.95 (0.81 to 1.10)1.16 (1.00 to 1.33)1.17 (1.02 to 1.35)0.91 (0.82 to 1.01)
 Combined oestrogen/progestogen0.87 (0.80 to 0.95)0.85 (0.76 to 0.95)1.09 (0.98 to 1.21)1.01 (0.91 to 1.11)0.87 (0.81 to 0.93)
Type of HC during current year of asthma exacerbation
 None11111
 Progestogen-only0.98 (0.93 to 1.02)1.02 (0.97 to 1.07)0.97 (0.94 to 1.01)0.98 (0.94 to 1.03)0.98 (0.96 to 1.01)
 Combined oestrogen/progestogen0.96 (0.92 to 0.99)0.95 (0.91 to 1.00)0.87 (0.83 to 0.91)0.99 (0.96 to 1.03)0.89 (0.87 to 0.92)
Duration of use of any HC (years)
 011111
 1–21.03 (0.99 to 1.07)0.96 (0.92 to 1.00)1.03 (0.99 to 1.06)0.99 (0.96 to 1.03)1.02 (0.99 to 1.05)
 3–40.92 (0.88 to 0.97)1.00 (0.95 to 1.05)0.92 (0.88 to 0.96)0.95 (0.90 to 0.99)0.93 (0.90 to 0.97)
 5+0.93 (0.89 to 0.97)0.92 (0.87 to 0.96)0.89 (0.86 to 0.93)0.98 (0.94 to 1.02)0.87 (0.84 to 0.90)
  • *All analyses were based on multilevel, mixed-effects Poisson regression that accounted for clustering of patients within general practitioner practices.

  • †Adjusted for follow-up time, level of adherence to asthma medication, age, smoking, Charlson Comorbidity Index, BMI, gravidity, any gynaecological condition and Index of Multiple Deprivation.

  • ‡Stratified analyses performed after interaction term between use of HC and BMI gave p=0.023 for any HC and BMI, p=0.000 for HC types and BMI and p=0.113 for duration of HC and BMI.

  • §Stratified analyses performed after interaction term between use of HC and smoking gave p=0.048 for any HC and smoking, p=0.107 for HC types and smoking, and p=0.000 for duration of HC and smoking.

  • BMI, body mass index; HC, hormonal contraceptive.