Magnesium citrate
"Having Opiate induced constipation and on methadone, bowel movements are a commodity and a gift. On my fourth day of no-pooping I began to panicky and read about MC.
Went to the pharmacy picked up the lemon lime flavour brand of Citrodan. 
I actually drank half the bottle on the way home and within 45 minutes I was on the toilet excreting the most foul smelling bowel movement I have ever had.
The next few hours were a bit uncomfortable ; gas, gurgling, running to the toilet and some cramping.
This stuff does what it is supposed to do with fervour and dedication.  Be prepared and stay in.  It's best to do it in the morning so you have all day to expel.  Drink lots of water throughout the day and wash .  It does smell bad coming out."