The causal role of smoking on the risk of headache. A Mendelian randomization analysis in the HUNT Study

Marianne Bakke Johnsen, Bendik K S Winsvold, Sigrid Børte, Gunnhild Åberge Vie, Linda Margareth Pedersen, Kjersti Storheim, Frank Skorpen, Knut Hagen, Johan Håkon Bjørngaard, Bjørn Olav Åsvold, John-Anker Zwart