The Biguanides are a group of oral anti-diabetic drugs.
It has been known since the Middle Ages that the French Lilac or Goat's Rue (Galega officinalis) is useful in the treatment of Diabetes Mellitus.
Modern chemical analysis revealed that the active chemical agent was a guanidine group. Purified guanidine was found to be rather toxic and so pharmacologists rushed to investigate derivatives. The first synthetic biguanides came into use in 1957 - Phenformin and Metformin (proprietary name : Glucophage).
Properly speaking these agents are not hypoglycaemics in that they will not lower blood sugar autonomously and are therefore relatively safe even at high doses. They act by reducing hepatic gluconeogenesis and enhancing peripheral sensitivity to the effects of insulin. The molecular mechanism is believed to involve AMP kinases.
The safety and generally mild side-effect profile of Metformin has made it a first-line agent in the treatment of Type 2 Diabetes Mellitus. Good adherence to treatment can drop glycosylated hemoglobin levels by around 2%. Biguanides are unusual for anti-diabetic agents in that they do not cause weight gain and can in fact assist weight loss. This combination of effects makes it useful in the treatment of Polycystic Ovarian Syndrome (PCOS).
Phenformin is little-used today because of its association with fatal lactic acidosis. This problem is associated to a much lesser extent with Metformin but nonetheless underlies some of the contraindications and side effects of that drug.
Patients with known renal, hepatic or pulmonary disease should avoid Metformin to avoid precipitating acidosis. The drug should also be stopped before patients receive intravenous contrast agents which are known to affect the kidneys.
More common side effects of Biguanide therapy include nausea, diarrhea, anorexia and an unpleasant metallic taste in the mouth.
Brunton et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics 11th edition.