it just seems not to last when in pain
Demerol has a half-life of 2.5 - 4 hours. That means 2.5 to 4 hours after you take it, the concentration of Demerol in your blood plasma is half of the dosage taken. Some of the medication will remain in your system for 5 to 6 times the half-life, or between 12.5 and 24 hours. I'm not sure exactly when Demerol reaches it's peak of concentration, but my guess would be between half an hour to an hour after taking it orally. After that, the concentration gradually begins to decline.