**Core Concept**
Metformin is a biguanide used in the treatment of type 2 diabetes mellitus. Its mechanism of action involves the inhibition of hepatic gluconeogenesis, which is the process by which the liver produces glucose from non-carbohydrate sources. Unlike other oral antidiabetic drugs, metformin does not stimulate insulin secretion and does not cause hypoglycemia.

**Why the Correct Answer is Right**
Metformin is preferred over phenformin due to its lower risk of lactic acidosis, a rare but potentially fatal complication associated with biguanide therapy. Phenformin, in particular, has a higher risk of lactic acidosis due to its greater potency and ability to inhibit mitochondrial respiratory chain complex I, leading to impaired oxidative phosphorylation and increased production of lactic acid. In contrast, metformin's effect on mitochondrial complex I is less pronounced, making it a safer alternative.

**Why Each Wrong Option is Incorrect**
**Option A:** Metformin has a faster onset of action than phenformin. This is incorrect because the onset of action of metformin and phenformin is similar, with both drugs typically taking several weeks to achieve their full effect.

**Option B:** Metformin is more effective than phenformin in reducing HbA1c levels. This is incorrect because while metformin is an effective oral hypoglycemic agent, its efficacy in reducing HbA1c levels is not significantly greater than that of phenformin.

**Option C:** Metformin has a lower risk of gastrointestinal side effects than phenformin. This is incorrect because both metformin and phenformin can cause gastrointestinal side effects, such as diarrhea and abdominal discomfort, although the incidence may be slightly higher with phenformin.

**Clinical Pearl / High-Yield Fact**
Lactic acidosis is a rare but potentially fatal complication of biguanide therapy, particularly in patients with underlying renal or hepatic disease. Patients taking metformin or phenformin should be monitored regularly for signs of lactic acidosis, such as nausea, vomiting, abdominal pain, and respiratory distress.

**Correct Answer: C. Lower risk of lactic acidosis.**