Pharmacokinetics
In the body
Oral onset about 30–60 minutes. Liver metabolizes it. Kidneys clear it. IV hits in minutes.
Timing
Oral morphine is only partly absorbed into the bloodstream because the liver metabolizes a large share on the first pass. That is why an oral dose is larger than an IV dose for a similar effect.
| Typical range | |
|---|---|
| Oral onset (immediate-release) | About 30–60 minutes |
| IV onset | Minutes |
| Duration (immediate-release) | About 3–4 hours |
| Half-life after IV | About 2 hours |
| Oral bioavailability | About 30–40% (first-pass in the liver) |
Metabolism and clearance
- The liver converts morphine mainly to morphine-6-glucuronide (still active for pain) and morphine-3-glucuronide (not useful for pain, can be neuro-excitatory).
- Kidneys clear those metabolites. Impaired kidney or liver function can make the drug last longer and hit harder.
- It crosses the placenta and is present in breast milk.
- It is only modestly lipid-soluble, so it enters the brain more slowly than drugs like fentanyl or heroin.