Calculating morphine doses

Oramorph breakthrough doses

(see also prescribing red flag medicines)

(this is also covered in year 3 primary care therapeutics and during palliative care)

Oral morphine is available in two forms:

Normal / immediate release tablets and liquid – would be expected to be effective after 20-30 minutes and to last up to 4 hours, e.g. oramorph® liquid
Modified /slow release (MR/SR), tablets, granules, or capsules – would be expected to last up to12 hours, e.g. MST® continus tablets, zomorph® capsules

Starting doses.

If the patient has been on maximum strength co-codamol (30/500mg two tablets qds i.e. 240mg codeine) then MST 15mg to 20mg every 12 hours is usually appropriate (codeine is approximately 1/10 strength of morphine i.e. 240mg is equivalent to 24mg)

Elderly or frail patients may require lower starting doses. Patients with renal failure will accumulate morphine metabolites and the dose and frequency should be reduced or a non-renally excreted alternative considered (e.g. fentanyl).

All patients on modified release morphine should have normal release morphine available when required for breakthrough pain, i.e.1/6th of their total 24 hour morphine dose.

For example a patient who is taking MST 30mg bd MST 30mg bd = 60mg oral morphine in 24 hours 60mg / 6 = 10mg oramorph, PRN, up to hourly if needed

Question: A patient taking MST 70mg bd requires how much oramorph?

Answer
(140mg/6 = 23.3mg, oramorph 10mg/5ml- would practically give a range of 20-25mg i.e. 10-12.5ml)
Some hospital trusts do not allow a range for strong opioids so you would either round down (20mg) or up (25mg). Please note that although oramorph® is a liquid it should always be prescribed in milligrams (mg) as there are two concentrations.