Table 5.

Relative risk of breast cancer in relation to smoking, by family history of breast cancer

	Family history of breast cancera
	No				Yes
	Cases	HRb	95% CI	P value	Cases	HRb	95% CI	P value
Ever previously smoked cigarettesc
Never	819	1.00	Baseline		239	1.00	Baseline
Ever	554	1.07	0.96–1.20	0.22	203	1.35	1.12–1.62	0.0018
	Interaction P = 0.038
Age at start of smoking, yearsc
Never smoker	819	1.00	Baseline		254	1.00	Baseline
< 20	427	1.17	1.04–1.32	0.0098	138	1.26	1.02–1.56	0.029
20+	96	0.82	0.67–1.02	0.076	55	1.56	1.17–2.10	0.0028
Age unknown	16	0.75	0.46–1.23	0.26	10	1.57	0.84–2.96	0.16
	Interaction P = 0.0029
		Trendd P = 0.034				Trendd P = 0.38
	Trend interaction P = 0.044
Starting smoking relative to age at menarche, years
Never smoker	819	1.00	Baseline		254	1.00	Baseline
Before or < 5 years	239	1.30	1.12–1.51	0.0006	61	1.06	0.80–1.41	0.68
5+ years	244	0.97	0.84–1.13	0.70	112	1.53	1.22–1.91	0.0002
Interval unknown	56	0.83	0.62–1.12	0.22	30	1.51	1.02–2.24	0.041
	Interaction P = 0.0001
		Trendd P = 0.0021				Trendd P = 0.40
	Trend interaction P = 0.0079
Cigarettes per day age among ever smokers, averaged over years when smokinge
Never smoker	819	1.00	Baseline		254	1.00	Baseline
1–4	185	1.01	0.86–1.18	0.95	73	1.32	1.01–1.71	0.040
5+	210	1.28	1.09–1.49	0.0019	69	1.42	1.09–1.86	0.010
Unknown	144	0.94	0.79–1.13	0.51	61	1.31	0.99–1.74	0.059
	Interaction P = 0.13
		Trendd P = 0.0019				Trendd P = 0.81
	Trend interaction P = 0.18
Duration of smoking among ever smokers, yearsf
Never smoker	819	1.00	Baseline		254	1.00	Baseline
1–9	129	0.94	0.78–1.14	0.54	48	1.20	0.88–1.64	0.24
10+	385	1.16	1.03–1.32	0.019	140	1.39	1.13–1.71	0.0020
Duration unknown	25	0.76	0.51–1.13	0.17	15	1.51	0.90–2.55	0.12
	Interaction P = 0.094
		Trendd P = 0.15				Trendd P = 0.86
	Trend interaction P = 0.37
Pack-years of smoking among ever smokerse
Never smoker	819	1.00	Baseline		254	1.00	Baseline
1 to < 10	208	1.02	0.87–1.18	0.84	77	1.28	0.99–1.65	0.060
10+	187	1.30	1.11–1.53	0.0015	65	1.48	1.13–1.95	0.0051
Unknown	144	0.93	0.78–1.12	0.46	61	1.30	0.98–1.72	0.069
	Interaction P = 0.16
		Trendd P = 0.055				Trendd P = 0.54
	Trend interaction P = 0.37

^aFirst-degree relatives, at recruitment

^bAdjusted for attained age (Cox regression time scale); time since recruitment to cohort (0, 1–2, 3+ years); birth cohort (1908–1939, 1940–1949, 1950–1959, 1960–1969, 1970–1996); benign breast disease (yes/no); socio-economic score (Acorn score as trend, missing); age at menarche (trend, missing); age at first pregnancy (trend, missing); parity (trend, missing); duration of breastfeeding (trend, missing); current oral contraceptive use before menopause (yes/no); alcohol consumption (never regular, trend current drinker 1 to < 60 g/day, current drinker ≥ 60 g/day, past drinker, drinker with unknown details); physical activity [log(metabolic equivalent) trend, missing]; pre-menopausal body mass index at age 20 years (trend, missing); post-menopausal body mass index (trend, missing); menopausal hormone therapy use (never used, ex-user, current oestrogen-only user, current oestrogen plus progestogen user, current user of other types, missing); menopausal status (pre- or post-menopausal) and age at menopause (trend, missing)

^cTime updated through follow-up

^dTrend excludes never smoker and unknown groups

^eTime updated at the point renewed information was available from follow-up questionnaire

^fTime updated in yearly steps