Opioids as MedicationsPage 8 of 17

6. Pharmacokinetics (ADME)

The absorption, distribution, metabolism, and excretion (known commonly as ADME) are important features of any medication. Absorption of oral opiates varies widely. Morphine is irregular in its oral uptake so it is usually given by IV or intramuscular injection (IM). There is, however, a slow-release oral form for those with chronic pain, though the patient must not chew or otherwise crush these tablets or an overdose will occur. Codeine is well absorbed if taken orally, and is usually used as a cough suppressant. Most opiates are subject to significant first-pass metabolism by the liver and are therefore less potent orally than if delivered by IV or injection.

The half-life of most morphine derivatives given by mouth is 3 to 6 hours, though some of the first-pass liver metabolites (after glucuronidation) still have considerable analgesic effects. Morphine is not well metabolized by neonates and is relatively long acting causing respiratory depression, so it should not be used during childbirth, where meperidine is a safer choice. Respiration can be restored with the opioid antagonist naloxone, as necessary.

For post operative or chronic pain, opioids have been used for patient-controlled analgesia (PCA). Patients receiving IV opioids are able to self-medicate via a pump (with dosing and limits determined by a physician’s orders). PCA allows patients to medicate as needed and avoids the potential delay due to other demands on the nurse providing pain medication. The pump has not generally led to overmedication and dependence as originally feared. Instead patients are treated more effectively and tend to self-taper their dosing as the pain resolves.

Acute overdose of morphine results in respiratory depression and even coma, and is characterized by constricted pupils. The antagonist naloxidone can be given by IV but this often leads to severe withdrawal symptoms if the overdose was, as is most common, in a habitual user. As with many other medications, the dosage of opiates for medication must be altered in any patients with reduced renal or liver function, which are especially common in elders with comorbidities.