Pharmacy Diabetes

B-2.4 Biguanides_Metformin-counselling intervention

Intermittent Diarrhoea

It is worth having a conversation with people who have been on metformin for long periods of time to understand if they experience intermittent diarrhoea. If people are experiencing intermittent diarrhoea, consideration should be given to metformin as the cause.  

  • If the intermittent diarrhoea is bearable and not inhibiting day to day life, then this may not require intervention.
  • If, intermittent diarrhoea is a concern for people, it is worth discussion with the prescribing doctor and/or referral to the health care team.

Metformin SR and MR are traditionally prescribed at night since the studies were conducted at this time of the day, however, the time of day is irrelevant. 

  • While not PBS listed to be taken twice a day (morning and night) this dosing is often better with respect to the gastrointestinal side effect profile.
Outer Shell Casing

Metformin SR or MR is designed to dissolve in the small intestine via small pellets that pass through the outer casing of the shell of the tablet/caplet. What is left to pass into the faeces is this outer shell. This often worries people since they will see the outer shell in the toilet bowl.  

  • It is often helpful to advise that the medicine itself is dissolving and working appropriately.
Impact on Blood Glucose

The effect on an individual’s blood glucose levels when beginning metformin is approximately  two weeks.  

If an individual has been asked to monitor their glucose levels at the start of metformin please let them know that metformin will take two to three weeks to impact this.
Temporary Discontinuation

Metformin should be ceased temporarily in times of illness, when having any form of dye injected for radiological procedures, during times of dehydration or fasting and before surgery (2 days prior, during and for 2 days after). This is to protect kidney function.  

Please refer the individual to their health care team.
Vitamin B12
  • Metformin is known for causing malabsorption of vitamin B12. This often presents as pins and needles and numbness which can be diagnosed as peripheral neuropathy.
  • It is now widely accepted that long term use of metformin will deplete vitamin B12 levels especially in susceptible individuals. Although the exact mechanism is still unclear, it is believed that metformin causes interference of absorption of the vitamin in the terminal ileum (small intestine). Supplementation with a multivitamin has shown to lower the prevalence of deficiency which ironically can present with symptoms similar in nature to nephropathy (nerve damage) and can be misdiagnosed in someone with diabetes (tingling in the hands and feet amongst other symptoms).
  • All individuals on metformin should be supplemented with sublingual or buccal vitamin B12 (not tablets since this will not be absorbed).li>
  • If there are no contraindications to vitamin B12 supplementation, consideration should be given for those on long term metformin. For further reading, please see,