Prescribing Opioids to Older Adults

Commonly Used Opioids

Table 1 lists the various properties of commonly used opioids, along with their indications and contraindications.

Codeine

Codeine is indicated for mild pain and may be used to step down from a stronger opioid in instances when a patient's pain is decreasing over time. A weak opioid, it has no analgaesic effect on its own, but is a prodrug, converted to morphine by the liver.^[8] Due to genetic differences, some people lack the ability to make this conversion due to low CYP2D6 enzyme levels;^[8] for them, an equianalgesic dose of another opioid should be considered if codeine appears to have no effect.

Morphine

In the oral form, morphine is six times more potent than oral codeine.^[9] Its metabolites are excreted renally.^[10] Many older adults have comorbid illnesses such as diabetes, hypertension, and congestive heart failure that predispose them to kidney disease. In addition, creatinine clearance decreases with age, even among healthy individuals.^[6] Therefore, a normal creatinine value may not guarantee that morphine metabolites will not accumulate, particularly at higher dosages. For this reason, morphine should be used with caution in the older adult population.

Hydromorphone

Five times stronger than morphine,^[9] hydromorphone also has renally cleared metabolites; however, owing to its higher potency, a much smaller dose can be used for an equianalgesic effect. This makes it a more suitable choice for patients with renal impairment. One advantage of hydromorphone is that it is available both orally and parenterally, allowing for use via either route without the need for converting between drugs.

Oxycodone

Oxycodone is twice as potent as morphine.^[9] Less than 15% of an oxycodone dose is excreted in the kidneys,^[10] making it an excellent drug for older adults. It also has a moderate side effect profile. It is available only orally; thus, patients who may require alternate routes throughout their course of treatment may benefit from another choice of drug.

Fentanyl

Transdermal fentanyl may be used for patients who do not tolerate the orally available opioids or who cannot swallow sustained-release formulations. Patches are available in multiples of 12.5 µg/h, but the recommended starting dose is usually 25 µg, the equivalent of a minimum equivalent daily dose of 50 mg of oral morphine.^[9] Because it can take up to 48 hours to attain full effects from the patch and another 48 to completely clear the drug upon discontinuation, fentanyl patches should only be prescribed to patients with stable opioid requirements. A conservative starting dose should be used, and there should be close monitoring for the first few days. Patients will still require a conventional as-needed breakthrough drug. Hydromorphone and oxycodone are the best choices.

Meperidine

There have been numerous warnings about the use of meperidine, especially for older adults, such that both the American Pain Society and the Institute for Safe Medication Practice do not recommend meperidine's use as an analgesic for chronic pain in this population.^[11] Its toxic metabolite, normeperidine, has an extremely long half-life and accumulates rapidly in patients with impaired renal function.^[12] In older adults with reduced creatinine clearance, this poses an even greater danger. Normeperidine has neuroexcitatory properties and can lower the threshold for seizures.^[13] Early evidence had provided an indication for meperidine in pancreatitis and biliary colic because of animal models which showed decreased sphincter of Oddi pressures compared with other opioids. However, human trials have since shown that meperidine is not superior to other opioids in avoiding smooth muscle spasm in biliary colic.^[14]

Combination Drugs

Codeine and oxycodone are both available in combination with acetaminophen, ibuprofen, or acetylsalicylic acid (ASA). Care should be taken when prescribing these. In the case of acetaminophen, it is important to ensure that the patient does not ingest more than 4 g/d of acetaminophen from all sources.^[15] In the case of ASA-containing formulations, care must be taken to make sure that there are no adverse renal or gastrointestinal (GI) effects. In patients taking cyclo-oxygenase 2 inhibitors, co-ingestion of ASA, even at lower dosages, can greatly increase the risk of upper-GI bleeding.^[16]

The nonopioid components of these combination drugs are not standardized and may vary with each formulation. It is, therefore, often preferable in older adults to prescribe the drugs separately rather than in combined form, so that the amount of each drug ingested can be monitored more accurately.

One common misconception is that the potency of the most popular codeine-containing formulation (codeine 30 mg) is equal to that of the most common oxycodone-containing medication (5 mg). Often these drugs are used interchangeably, especially by patients; however, closer scrutiny reveals that the oxycodone-containing drug is twice as potent as its counterpart. Therefore, when switching between these medications, it is important to ensure that old pills are disposed of safely.

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