Association of candidate genetic variants with number of injection. OR = Odds ratio; CI = Confidence interval *: Major allele homozygote / Heterozygote / Minor allele homozygote **: Uncorrected P-value from linear regression model. When applying Bonferroni correction, uncorrected P-value for significance should be less than 0.0031. †: Analyzed under dominant model because of extremely low proportion of minor allele homozygotes ††: ε2+ = ε2 carrier, ε4+ = ε4 carrier. To access the data, click or select the words “Appendix 4.”