Introduction

When it comes to any pharmacy licensing exam – including the NAPLEX – it’s vital that you have a comprehensive understanding of each major drug / drug class. Here, we focus on the fundamental facts about metformin you need to know.

First discovered in the early 1920s, it was introduced to the US in 1995 to control blood sugar levels in patients with type 2 diabetes. The drug has since gone on to join the WHO List of Essential Medicines and, as of 2016, remains the fourth most prescribed drug in the United States.

Indications

Metformin is used in the treatment of the following indications:

• Diabetes type 2: a first-line agent to control blood glucose levels. It may be used singly or in combination with oral anti-diabetic medicines.
• Polycystic ovarian syndrome (PCOS): metformin is used here to lower blood sugar levels as well as to stimulate ovulation and lower the risk of miscarriage. Metformin also carries the health benefits of reducing high cholesterol levels and the risks associated with cardiovascular disease.

Metformin may also be used for purposes not listed in this guide.

Mechanism of action

Metformin is a member of the biguanide class of oral hypoglycemic drugs.

At the most basic level, it works by increasing insulin sensitivity. In reality, then, metformin works to achieve the following effects:

• Inhibit glycogenolysis and gluconeogenesis
• Increase glucose uptake by skeletal muscle and other tissues
• Inhibit intestinal glucose absorption – reducing glucose uptake

Metformin does not increase pancreatic insulin secretion.

Dosage

Metformin is not a drug administered at any fixed dose for a prolonged period. Instead, the dose must be increased gradually – not least to reduce the risk of developing gastrointestinal side effects.

• Standard initial adult (immediate-release) dose: 500mg per day.
• Titration: This dose is then titrated up by at least 500mg per week per dose to 500mg three times daily, or 850mg twice daily.
• Maintenance dose: 2,000mg daily in divided doses
• Maximum dose: 2,550mg per day.

Metformin should be taken with a full glass of water with or after food.

Metformin is contraindicated in patients with a renal function of 30mL/min/1.73m², or less.

The drug is not recommended for patients with liver damage due to the risk of lactate accumulation.

Side effects

Side effects associated with this medicine include:

• Gastrointestinal upset – nausea, vomiting, diarrhea
• Metallic taste
• Rarely, lactic acidosis
• With long-term use, vitamin B12 deficiency

Lactic acidosis risk increases in patients with pre-existing hepatic impairment or in patients who present with acute alcohol intoxication.

Clinical factors

Here are some of the essential factors to consider when prescribing metformin:

• Avoid when patients must undertake IV contrast media use – for example – in cases of CT scans or coronary angiography. Taking it during this period increases the risk of renal damage, metformin accumulation and lactic acidosis.
• Drugs known to impair renal function – NSAIDs and ACE inhibitors, for example – should be used with great caution with metformin due to the risks outlined above.
• Many drug classes oppose its effects by increasing blood glucose levels. These drug classes include loop diuretics, thiazide diuretics and prednisolone (steroid-induced diabetes).
• The drug should be taken with or after food to reduce gastrointestinal side effects.
• Unlike other oral antidiabetic drugs, it is not associated with weight gain. Instead, many patients experience weight loss. Metformin is also associated with a lower risk of hypoglycemia.

Metformin remains a very important medicine. With the ensuing rise of type 2 diabetes throughout the United States and much of the developed world, a drug that controls blood glucose levels, whilst maintaining a modest side effect profile and control of blood lipids, metformin remains one of the most important medicines of today. For the clinician and healthcare provider, an intimate knowledge of metformin is an absolute must.

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