Methadone dosage may need to be increased by around 22% a week after starting efavirenz treatment, according to Dr Fiona Mulcahy of St James's Hospital, Dublin.
Elaborating on data first presented at the Seventh Conference on Retroviruses and Opportunistic Infections, Dr Mulcahy told a British HIV Association conference satellite meeting organised by Du Pont Pharma that efavirenz reduced methadone exposure (AUC) by an average of 60% within 24 hours of the first efavirenz dose.
This occurs because efavirenx speeds up, or induces, the metabolism of methadone. However, dosage adjustment should not take place immediately.
"Complaints about withdrawal symptoms during days one to three of efavirenz treatment are more likely to be connected with the neurological effects of efavirenz. The induction effect of efavirenz tends to build up over a week or more, so there will not be an immediate reduction in methadone levels" said Dr Mulcahy.
The methadone dose increase required will depend on the existing methadone dose. People taking high doses of methadone will need correspondingly larger increases in their methadone script, which should be built up day by day as soon as withdrawal effects begin to be noticed. In her Dublin clinic, Dr Mulcahy has been increasing dosages by 10mg at a time until withdrawal symptoms stop.
Methadone levels need to be reduced again if efavirenz treatment stops for any reason, and the dose reduction will need to be tapered over 8-10 days to avoid withdrawal symptoms.