Undesirable effects, such as nausea or drowsiness, can occur on starting an opioid or with a dose increase. These may require the use of additional medication, such as anti-emetics, but generally resolve after about 1 week. Occasionally, the dose of opioid may need to temporarily be reduced and titrated up again more slowly. If severe or persistent undesirable effects occur, seek specialist advice, as an alternative opioid may be required.

Oral morphine to another oral strong opioid

• CKS recommends that primary care healthcare professionals seek specialist advice, or consult local guidelines (where available), when selecting the opioid and dose to switch to because experience in primary care is likely to be limited and alternative oral opioids are best initiated by a person with experience in palliative care. Methadone should only be initiated by a specialist because it has a long and unpredictable half-life, with considerable inter-individual variation, and requires careful monitoring.
• There are differences in opinion regarding dose conversion ratios — current practice for converting opioid doses is based on pharmacokinetic drug data (such as bioavailability after oral administration) from observational and uncontrolled studies, and on expert opinion and experience. Reported equi-analgesic dose ratios vary widely among strong opioids. When converting from one opioid to another, regular assessment and reassessment of efficacy and adverse effects is essential because of the lack of evidence on equi-analgesic dose ratios and inter-individual variations. Most specialists would advocate applying the calculation and then allowing a relative dose reduction in case there is incomplete cross-tolerance, which would result in a much greater clinical effect than anticipated.
• Particular attention to monitoring and dose titration up or down is needed when:
• Switching between opioids at high doses.
• There has been a recent rapid escalation of the first opioid.

Oral morphine to subcutaneous diamorphine or morphine

• In primary care, when switching the route of administration of one strong opioid to another, the most common switch is from oral morphine sulphate to subcutaneous diamorphine or morphine.
• Diamorphine is much more soluble than morphine and therefore easier to administer in higher doses. It is also compatible with most other drugs which may need to be administered by a subcutaneous infusion. However, morphine is an alternative, and most people do not require doses large enough to cause solubility issues:
• Parenteral diamorphine is approximately three times as potent as oral morphine, so the total daily dosage of oral morphine should be divided by three to obtain the 24-hour subcutaneous dose of diamorphine. See Table 1.
• The oral to subcutaneous potency ratio of morphine is between 1:2 and 1:3 (that is, the subcutaneous dose is one third to one half of the oral dose). In practice, most centres divide the oral dose by two and re-titrate as necessary. See Table 1.

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