I heard oxycodone is stronger than morphine
Well you heard wrong. Orally yes, it takes less oxycodone to achieve the amount of pain relief that morphine can achieve at say 30 mg.
But morphine is more potent (2x more potent, infact).
When the drugs are delivered through a route that can deliver 100% of the drug to the brain (aka IV), morphine wins hands down.
The difference in oral has NOTHING to do with potency of the drugs, it has everything to do with metabolism. Oxycodone's oral BA is roughly 87%, while morphine's is much more variable (some sources say 10% and yet others say 30% ). We'll just say morphine's oral BA is 25%.
Now Patient A is seen in the ER complaining of severe lower back pain. The doctor prescribes A 10 mg oxycodone hydrochloride (no APAP, aspirin or anything - just plain old oxycodone). Patient A will get about 87% of that 10 mg (so 8.7 mg) crossing his BBB and achieving him great pain relieve.
Patient B is presented to the ER with the same problem (lower back pain). The doctor prescribes B only 10 mg morphine sulfate to be taken through the oral route). Patient B will be getting about 25% of that 10 mg (so 2.5 mg) crossing his BBB and achieving him very little pain relief.
Do you see the difference now? It's not a potency issue (morphine is without a doubt the more potent drug overall), but it's a metabolism issue in this case. Oxycodone is more orally active than morphine, but it is not more potent. Morphine is the more potent drug.
When the two drugs are delivered through IV (where both cross the BBB at a 100% rate), it takes 2 times as much oxycodone to achieve the same amount of pain relief as morphine 10 mg.
Oxycodone IV 20 mg = Morphine IV 10 mg (the reason why oxycodone is not a feasible IV drug - at that dosage there would be too many side effects).
