Bisacodyl
"I've had chronic constipation issues my whole life (in my early 30s) and this is literally the only thing that will basically clear me out. I only really use it when my constipation is particularly bad - it's one of those things where you really have to make sure you are very close by to a bathroom for at least 24 hours after taking it. It's definitely not the most comfortable experience - you will get stomach cramps and be up on the toilet for most of the night. But it's definitely effective. To me it is worth it  to get cleared out and not have to deal with the nausea and pain that comes from being severely constipated."