Codeine exerts its effect through conversion to morphine anyway so it should work well. I think in clinical practice (at least in national guidelines in Norway) 5mg morphine is said to equal 500 mg paracetamol and 30 mg codeine. In pharmacoepi-studies of opioid use I have used 1 mg codeine = 0.15 mg morphine. However as we have access to cheap codeine in Norway morphine is rarely used until after weak opioids have been tried so I can not speak from any real life knowledge.

If cost is an issue, a short term small doses of morphine combined with NSAIDs should work.

we did a fairly comprehensive literature search when we did the Nice Guideline Opioids in Palliative Care. If you trawl through the document you will see some recommendations but also some background and review on the papers looking into opioids.

http://www.nice.org.uk/guidance/cg140/resources/guidance-opioids-in-palliative-care-safe-and-effective-prescribing-of-strong-opioids-for-pain-in-palliative-care-of-adults-pdf

Codeine, which in itself possesses no analgesic activity, is metabolized to morphine by CYP450 2d6, which is expressed in subnormal levels in 5-10% of humans. They will not benefit from codeine. One could reasonably argue if there is any advantage in using a drug with such a high failure rate at all.

Also, some patients are rapid metabolizers and will experience high plasma morphine concentrations. This had led to at least two deaths of babies nursed by codeine users.

The rational choice, as Francis suggests, is to use morphine in lower doses. Same or better effect, fewer side effects, less risk to the patient.

For acute surgical pain any type of opiods will work well together with paracetamol and NSAIDs. For a chronic non malignant pain you need to be very careful.