Web site: September 2010
Prescriber Update 2010;31(3):25
Healthcare professionals are advised that the Apotex brand of metformin 500mg and 850mg tablets are not scored and cannot be accurately halved. This advice follows recent reports sent to CARM describing problems halving the 500mg tablet.
The recommended starting dose for metformin is 500mg once or twice daily; however Medsafe is aware that some patients are being prescribed a starting dose of 250mg to minimise gastrointestinal side effects. Diarrhoea, nausea and vomiting occur in more than 10% of patients starting metformin but usually resolve over a short period of time.
Medsafe has obtained expert advice on metformin dosing and has been informed that a 250mg daily dose is likely to be subtherapeutic. This means any small differences in dose resulting from inaccurate halving of a 500mg tablet are unlikely to be clinically relevant. Similarly, any small differences from inaccurate halving are unlikely to affect the development of gastrointestinal tolerance.
Prescribers are also advised to consider the difﬁculties associated with halving a 500mg tablet and remember that an 850mg tablet is available before titrating metformin doses in increments of 250mg.
Further information on metformin dosing can be found on the Medsafe website at: http://www. medsafe.govt.nz/profs/Datasheet/dsform.asp