Fourthly, a possible limitation of Mendelian randomization studies is a potential association of an indirect variant (ADH1B in our study) with non-endogenous exposures.
Fourthly, a possible limitation is that the reader was able to partially identify which mammograms had cancer at baseline.
Fourthly, it is possible that stopping smoking results in some kind of physiological disturbance which precipitates an illness episode.
Fourthly, it is possible that our results are only representative of Norwegian values and would not reflect preferences in other countries.
Fourthly, it is possible that other factors external to the dimensions of the instruments may have confounded some of the hypothesised relationships.
Fourthly, it is possible that TCR/pMHC interactions may be differentially affected by the mechanical stress induced by T cell-APC interaction [ 23].
Fourthly, it is possible that the use of other more effective chemotherapy regimens could provide additional survival benefit, such as induction TPF (docetaxel, cisplatin and fluorouracil) [ 33, 34] and PX (cisplatin and capecitabine) [ 35].
Fourthly, it might also be possible that adolescents who are already depressed and anxious might be more likely to adopt poor dietary habits, physically inactive lifestyle, involved in smoking and other tobacco products use.
Fourthly, we did not evaluate all possible interventions in cancer control, and our selection of interventions for analysis was pragmatic and somewhat arbitrary.
Fourthly, studies used different exposure assessment, implying possible misclassifications.
