Table 3:

Association between antibiotic exposure and risk of attention-deficit/hyperactivity disorder in the sibling cohort (n = 64 019)

Variable	No. of events	Person-years^*	Type of analysis; HR (95% CI)
Variable	No. of events	Person-years^*	Unadjusted	Adjusted^†
Main analysis
Prenatal antibiotic exposure
Unexposed	2913	360 947	1.00 (Ref.)	1.00 (Ref.)
Exposed	3112	326 402	1.07 (1.01–1.14)	1.06 (0.99–1.13)
Secondary analyses
Exposure by trimester^‡^§
None	2913	360 947	1.00 (Ref.)	1.00 (Ref.)
First	1446	138 371	1.02 (0.94–1.11)	1.03 (0.94–1.13)
Second	1506	145 603	1.13 (1.04–1.23)	1.10 (1.00–1.20)
Third	1258	131 952	1.02 (0.93–1.11)	1.02 (0.93–1.12)
Antibiotic class^§
None	2913	360 947	1.00 (Ref.)	1.00 (Ref.)
Penicillin	2063	215 600	1.01 (0.94–1.09)	1.03 (0.95–1.11)
Non-penicillin β-lactams	582	55 855	1.18 (1.04–1.34)	1.09 (0.96–1.25)
Macrolides and related antibiotics	700	55 595	1.12 (0.99–1.26)	1.07 (0.95–1.21)
Other	784	77 095	1.01 (0.90–1.13)	1.01 (0.89–1.13)
No. of antibiotic courses
0	2913	360 947	1.00 (Ref.)	1.00 (Ref.)
1	1921	223 769	1.05 (0.98–1.13)	1.04 (0.96–1.12)
2	750	68 775	1.10 (0.98–1.28)	1.11 (0.99–1.25)
≥ 3	441	33 859	1.12 (0.97–1.30)	1.10 (0.95–1.28)
Cumulative antibiotic duration, d
0	2913	360 947	1.00 (Ref.)	1.00 (Ref.)
1–7	1233	146 636	1.06 (0.97–1.16)	1.04 (0.95–1.15)
8–14	1039	110 020	1.04 (0.94–1.14)	1.03 (0.94–1.14)
> 14	840	69 746	1.14 (1.02–1.28)	1.13 (1.01–1.27)

Note: CI = confidence interval, HR = hazard ratio, Ref. = reference.
↵* Values for person-years were rounded to the nearest integer.
↵† Adjusted for sex, mother’s age at delivery, birth order, year of birth and prenatal exposure to antidepressants.
↵‡ First trimester = 0–13 weeks of pregnancy, second trimester = 14–27 weeks, third trimester = 28 weeks until date of birth.
↵§ A child could be exposed to antibiotics during multiple trimesters and could be exposed to multiple antibiotic classes.