Close up of nurses hands drawing up an injection from a vial into a syringe

Switching routes - oral to injection

Opioids should be given by mouth unless there is a compelling reason not to do so. If the oral route is not viable (for instance the patient is vomiting or is unable to swallow) they are usually switched to a subcutaneous injection.

Regular or “as required” opioid injections are equivalent to immediate-release (IR) oral opioids in terms of duration of action. If your patient is taking a modified release (MR) oral opioid for background pain this must be converted to a continuous subcutaneous infusion (CSCI). Without this pain may return and your patient may experience opioid withdrawal. Subcutaneous infusions are usually administered over 24 hours and are often referred to as a “syringe driver” after the device that is used to deliver the infusion.

Calculating the dose of the opioid injection

Opioid injections are more potent than oral preparations because they do not undergo first pass metabolism. This means the dose of the injection will be different to the oral formulation. Morphine injection, for instance, is twice as potent as oral morphine so the dose must be halved.

Prescribing a continuous subcutaneous infusion (CSCI)

Because CSCIs are usually set to deliver the opioid drug over 24 hours, the total daily amount of oral opioid is first calculated. The oral to injection conversion factor is then used to determine the dose of injection. For morphine, the oral dose is haved. This ratio is different for each opioid and we recommend you seek specialist advice for opioids other than morphine.

Consult your local guidance and/ or seek specialist advice for oral to subcutaneous conversion ratios.

Starting the infusion

If the patient is only taking an immediate-release oral opioid, the infusion can be started immediately (or as soon as is reasonably practicable). If the patient is taking a modified-release opioid, the infusion should start 2 hours before the next oral dose would have been due, so as not to risk overdose. If the previous dose has been missed, the infusion can be started immediately.

Switching from infusion to oral opioid

The dose needs to be recalculated, taking account of the injection: oral conversion factor for the opioid concerned. Consult your local guidance and/ or seek specialist advice for oral to subcutaneous conversion ratios.

The first oral dose is given at the same time as the infusion comes down. For patient safety, this should be planned ahead to take place at the time the patient will usually take their oral MR opioid.

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Opioid analgesics: Treating Pain in People with Cancer

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