Name a common side effect of metformin.

Metformin is well-known for its association with gastrointestinal upset as a common side effect. This can manifest as symptoms such as nausea, diarrhea, and abdominal discomfort. The mechanism behind this side effect is likely related to the way metformin affects gastrointestinal motility and the absorption of glucose in the intestines. While this side effect can be bothersome for some patients, it often diminishes over time as the body adjusts to the medication.

The other options do not align with the typical side effect profile of metformin. Insomnia is not commonly associated with this medication, and hyperglycemia is generally a concern in the context of inadequate diabetes management rather than a direct side effect of metformin. While weight loss can occur in some patients taking metformin, it is not classified as a side effect in the same way gastrointestinal upset is, but rather a potential benefit for those who are overweight or obese.