Morphine is a highly potent and addictive opioid that is used widely in clinical medicine and surgery for a variety of reasons. Due to the euphoria and pleasurable effects the drug exerts, it is also widely used for recreational purposes and is commonly abused by drug addicts.
Pharmacokinetics of morphine
Morphine is absorbed into the bloodstream via multiple routes of administration including:
Orally in the form of tablets or capsules.
Rectally inside suppositories or gel-coated capsules that can be inserted into the anus so the drug can be absorbed into the rectal blood vessels.
Intravenously using a hypodermic needle to inject the drug directly into the bloodstream. This is one of the fastest ways to feel the effects of the drug and is therefore one of the most common routes of administration.
By subcutaneous injection where the drug is injected into the subcutaneous tissue just underneath the top layer of the skin. This creates a depot from which morphine can be rapidly delivered into the bloodstream.
Morphine can also be injected into the spinal column for spinal or epidural anesthesia.
Morphine can also be inhaled or snorted in powder form.
Metabolism or breakdown
Once morphine reaches the bloodstream, it is carried to the liver where a large proportion of it is broken down, a process termed first-pass metabolism. Due to this first-pass metabolism, only around 40% to 50% of morphine reaches the central nervous system.
In the case of subcutaneous injection, blood levels of morphine peak after about 20 minutes and with oral ingestion, levels peak after about 30 minutes.
During metabolism, around 60% of morphine becomes morphine-3-glucuronide and 6% to 10% becomes morphine-6-glucuronide. The morphine is glucuronidated by the phase II metabolism enzyme UDP-glucuronosyl transferase-2B7. The drug may also be metabolized into small quantities of normorphine, codeine, and hydromorphone.
Around 90% of morphine taken is excreted from the body within 24 hours, mostly in the form of urine. Morphine has an elimination half-life of around 120 minutes. The drug can be stored in fat, so remains detectable for a long time after use and even after a person has died.
Reviewed by Sally Robertson, BSc