Switching between oral and parenteral dexamethasone

Both dexamethasone tablets & injection consist of the salt dexamethasone sodium phosphate. Doses are prescribed and given as dexamethasone base.  Patients may at some point require to be switched from oral to parenteral route of administration or vice versa.

Dexamethasone injection is available as  dexamethasone base 3.3mg/ml.

Dexamethasone tablets are available as 0.5mg, 2mg, 4mg & 8mg

There needs to be clarification as to local policy for the conversion of dosage between different routes.

Dexamethasone given oral is approximately 80% bioavailable; where as when given parenteral  is 100% bioavailable.

The Scottish Palliative Care Guidelines provide the following advice:

For pragmatic purposes, 4mg oral dexamethasone is approx. equivalent to 3.3mg parenteral dexamethasone.

However, some centres continue to use a 1:1 conversion i.e. use the same dose independent of whether oral or parenteral administration.

It is recommended for consistency & to avoid confusion within an organisation that all prescribing should agree to adhere to the same conversion policy.

From consultation with staff from the palliative care service it was felt that using the 1:1 conversion would allow for easier measurement of doses – particularly where patients were prescribed doses of 0.5mg or less.

Using the 1:1 ratio also allows for prescribing to include multiple routes of administration option on the one medicine chart entry – which would reduce the likelihood of a patient receiving a dose of dexamethasone by 2 different routes at any one time if they were prescribed separately.

When converting between oral and parenteral administration of dexamethasone a 1:1 conversion will be used.

The table below gives examples of the volume of injection needed to provide the required dose. This can be extrapolated for higher or lower doses.

Scottish Palliative Care Guidelines http://www.palliativecareguidelines.scot.nhs.uk/