Opiates - The Speed of Occurrence

Morphine on one end has relatively low lipid solubility and is a much slower drug to onset. Only about 10 to 20 percent of morphine is unionized at physiologic pH so it doesn't diffuse into tissues very quickly. Meparidine is metabolized also in the liver to a main metabolite that is known as normaperidine which can actually cause seizures especially in renal failure where that metabolite can accumulate. Sufentanyl is similar to fentanyl but more lipid soluble than Fentanyl so faster onset than Fentanyl. And then remifetinol of course is a different structure completely.