Doxazosin and ACEI dose equivalence

To community pharmacists in EHS CCG / H&R CCG areas

A message from EHS and H and R medicines management team:

This message has also been sent to all GP practice Prescribing Leads for info.

Due to the current supply problem with doxazosin mr, pharmacists are advising GPs of dose equivalences when switching to plain doxazosin.

However, an equivalent dose switch is not appropriate for many patients for the following reasons.

We have previously agreed with local cardiologists that the best initial switch when changing from doxazosin mr to plain is to halve the dose to reduce the risk of postural hypotension i.e. doxazosin 8mg mr switched to 4mg plain, with subsequent monitoring and titration as needed. The rationale for this decision was the fact that the total bioavailability of mr tabs is less than the plain tablets – approx 50-60% of the plain tabs according to the SPC of Cardura XL, but other sources vary; cardiologist experience also suggested more caution when using the plain tablets.

Cardura XL SPC extract:

At steady-state, the relative bioavailability of doxazosin from Cardura XL compared to immediate release form was 54% at the 4mg dose and 59% at the 8mg dose.

Therefore, any recommendations from pharmacists to switch mr to plain at an equivalent dose conflicts with our previous advice, and we would ask pharmacists to advise a switch to half the dosage.

Below is a document from UKMI that may be of interest.

Doxazosin dose conversion Guys aug05

ACEI dose equivalence

On a similar note, we have also had a report of a recommendation to change ramipril 2.5mg to enalapril 2.5mg – these are obviously not therapeutically equivalent doses. Attached is a doc which includes some approx dose equivalence info (table 1 on page 3 probably of most use).

ACEI_ARB comparisons LNDG Jan 2008