NHS Greater Glasgow & Clyde Area Drug and Therapeutics Committee
Greater Glasgow and Clyde Medicines
Medicines Update

Mercaptopurine: safe prescribing

Mercaptopurine is an oral immunosuppressant used in the management of Crohn’s disease at doses ranging from 1 - 1.5mg/kg daily for adults (although some patients may respond to lower doses).  Higher doses are required when mercaptopurine is used in the management of acute leukaemias (initial daily adult dose of 2.5mg/kg).


Case report:

A hospital letter recommended ‘6-mercaptopurine 50mg daily’ which was interpreted by the GP practice as a total daily dose of 300mg.  The community pharmacist, on receiving a prescription for 300mg daily, queried the dose and it was changed to 50mg daily. The patient received the correct dose.


Historically, practitioners involved in the prescribing of this immunosuppressant have commonly referred to it as ‘6-mercaptopurine’ or ‘6-MP’.  However, it is recognised that as use of the numerical prefix is a risk which can lead to dosing errors. It should be prescribed simply as ‘mercaptopurine’.  


Key messages:

  • Practitioners not routinely involved in the prescribing of mercaptopurine may be unaware that a number prefix may be used.  To prevent confusion and errors, prescribers are advised to use the approved name which is mercaptopurine.
  • Where possible, establish the indication, patient’s weight and mercaptopurine dosing history.
  • High doses should always be discussed with the prescriber:  300mg is a high dose regardless of the indication.