The Pharmaceutical Journal
Vol 276 No 7390 p263
4 March 2006

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· Methadone mixture
· Brand prescribing
· Oxygen services
· BuTrans and the SMC
· The profession (4)
· The Society
· Reciprocity
· CPD

Letters to the Editor

Methadone mixture

Society’s guidance is too prescriptive

From Mr M. Bennett, FRPharmS

I was pleased to read (PJ, 25 February, p245) that the Royal Pharmaceutical Society’s Council has made an exception in allowing the extemporaneous preparation of methadone mixture 1mg/ml, despite the fact that products with a marketing authorisation exist. This makes sense given the problems with Controlled Drugs storage because the alternative could have severely reduced the number of patients that some pharmacies can treat.
I was less impressed with the need to go into such detail regarding the method of manufacture. Obviously, safe methods have to be employed with a proper audit trail, but I fear the suggestion that no reliance can be placed on the quantity stated on manufacturers’ packs will prove counter-productive and will potentially increase the risk to patients. I looked into this in 2002 and I spoke to the pharmacist at one of the companies that distribute the powder and green syrup to check the tolerances that they work to. He assured me that:

· 1g bottles of methadone powder contain between 997mg and 1,003mg; this is critically important to the company because it is equally bound by the CD legislation

· 1L bottles of green syrup are filled by weight to give a target volume between 1,005 and 1,015ml

So a worst-case scenario would result in 997mg being dissolved in 1,015ml at one end and 1,003mg being dissolved in 1,005ml at the other, providing 0.9823mg/ml and 0.998mg/ml, respectively. Discarding 10ml of the green syrup before adding the methadone would result in a more accurate product with a variance of 997mg/1,005ml = 0.992mg/ml at the low end and 1,003mg/995ml = 1.01mg/ml at the high end. Surely this is accurate enough, particularly when linked with recording of batch numbers so as to to relate back the final product to the original manufacturers’ packs. After all, we are accepting that the powder is what the manufacturer claims it to be — methadone. We do not analyse that.

It is my belief that the risks involved in manufacturing 1mg/ml methadone mixture are less if you reconstitute this by adding a manufacturer’s sealed 1g of methadone powder to a manufacturer’s sealed 1L of green syrup, recording batches etc, than if you attempt to follow the Society’s guidance in weighing and measuring both the powder and the liquid.

The significant errors I have come across in the past are where pharmacists have managed to add 10g of methadone to 1,000ml of syrup and inadvertently dispense 10mg/ml for 1mg/ml. That error is almost impossible with the method stated above, but much more likely when the pharmacist (or a delegated member of staff) weighs from, say, a 25g or 50g container of methadone powder and makes the mistake of placing a 10g weight rather than a 1g weight on the scales.

I would ask the Practice Committee to reconsider the detail within the guidance provided. Perhaps a statement that “robust standards and systems must be in place to ensure the quality of extemporaneously prepared methadone mixture so that patient care is not compromised” is all that is required along with some examples of how this might be achieved, rather than the prescriptive text that has been produced.

Martin Bennett
Managing Director
Associated Chemists (Wicker) Ltd
Sheffield

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