User blog comment:TRAPPER/How many years do you think luffys life has been shortened/@comment-3242083-20110114003310/@comment-1595565-20110114003948

Technically he didn't take the treatment twice. In Impel Down he took a combination of Vigor and Healing Hormones while at Marineford he only took Vigor Hormones. While I'm sure it still wasn't good for his body, his lifespan might not have lowered at Marineford.