While there's no peer-reviewed data on why this happens, according to Rob Stransky, president of the telehealth provider NiceRx, this can happen because of how these drugs slow the body's gastric emptying process, or how food moves away from the stomach into the small intestine
[84] Hamilton-Reeves J, Rebello S, Thomas W, et al
The pre-operative level of the oxidative stress markers (MDA and SOD) and the apoptosis markers (fas and caspase-3) also did not show any significant difference between the control and L-carnitine groups
Nat Chem Biol 10:917 Hwang C, Sinskey AJ, Lodish HF (1992) Oxidized redox state of glutathione in the endoplasmic reticulum
Most of the drug is processed by the liver and excreted through urine, with its pain-relieving effects lasting around 4 to 6 hours