US study warns against brand switching
Switching between branded and generic phenytoin products can lead to a loss of seizure control in patients with epilepsy, a US study has shown (Neurology 2004;63:1494).
Researchers measured changes in total and free phenytoin concentrations
in patients whose seizure control had worsened following a change to
their prescribed medicine. The change had followed implementation of
a mandatory switching policy from branded to generic product.
The researchers found that substitution of branded for generic phenytoin
or vice versa was associated with an approximate 30 per cent decrease
in both total and free phenytoin concentrations.
In terms of clinical practice in the UK, new guidance from the National
Institute for Clinical Excellence states that the formulation or brand
of anti-epileptic drugs should not be changed. The British National Formulary
also advises that there may be a pharmacokinetic basis for maintaining
a particular brand of phenytoin in some patients. However, it states
that single dose tests do not reveal clinically relevant differences
in bioavailability between available phenytoin sodium tablets and capsules.
The US researchers warn, however, that single dose tests, which can suggest
bioequivalence between products, are not conducted under conditions encountered
in clinical practice. |
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