Smoking and risk of breast cancer in the Generations Study cohort

Table 4 Relative risk of breast cancer in relation to ever smoking, by level of alcohol consumption

Strata	Total number of cases in strata	HR for ever smoking relative to never smoking within strata of alcohol consumption^a	95% CI	P value
Alcohol^b
Non-drinker	110	0.97	0.61–1.52	0.89
Ever drinker	1705	1.18	1.07–1.30	0.0010
		Interaction P ^c = 0.41
Alcohol^b
Non-drinker	110	0.97	0.61–1.52	0.89
< 20 g/day	706	1.17	1.00–1.36	0.049
20 to < 40 g/day	356	1.17	0.95–1.44	0.13
40 to < 60 g/day	87	1.12	0.73–1.72	0.61
≥ 60 g/day	27	1.92	0.78–4.76	0.16
Current, amount unknown	180	1.47	1.09–1.98	0.011
Former drinker	295	0.98	0.78–1.25	0.89
Drinker, details missing	54	0.73	0.38–1.42	0.35
		Interaction P ^c = 0.33

^aAdjusted 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); family history of breast cancer in first-degree relatives (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); 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)
^bTime updated through follow-up period
^cInteraction across all categories, including missing groups