Thiazolidinediones (pioglitazone, rosiglitazone) improve insulin sensitivity by activating peroxisome proliferator-activated receptor gamma, a nuclear receptor that regulates the transcription of genes involved in glucose and lipid metabolism. They are the only oral agents that directly address peripheral insulin resistance at the nuclear level rather than stimulating insulin secretion or blocking glucose absorption.

Peroxisome proliferator-activated receptor gamma is most highly expressed in adipose tissue, where its activation promotes differentiation of preadipocytes into small, insulin-sensitive adipocytes, redistributes lipid from visceral to subcutaneous depots, and increases expression of glucose transporter type 4 and adiponectin. The net effect is improved insulin signaling in adipose tissue, muscle, and liver, reducing fasting and postprandial glucose without stimulating insulin secretion. Because they require transcriptional changes to take effect, thiazolidinediones have a slow onset — full glucose-lowering effect may take 6 to 12 weeks.

Pioglitazone is the primary thiazolidinedione in clinical use. Rosiglitazone was associated with increased risk of myocardial infarction in meta-analysis and now carries severe prescribing restrictions in most markets. Pioglitazone, by contrast, showed modest macrovascular benefit in the PROactive trial (reduction in the composite secondary endpoint) and is also approved for nonalcoholic steatohepatitis in some guidelines, where it reduces hepatic inflammation and fibrosis.

Alpha-glucosidase inhibitors (acarbose, miglitol) work at the intestinal brush border to slow the digestion of complex carbohydrates, reducing the rate of glucose absorption after meals. They lower postprandial glucose excursions without stimulating insulin secretion and carry no intrinsic hypoglycemia risk as monotherapy.

Alpha-glucosidases are brush border enzymes (sucrase, maltase, glucoamylase) in the small intestinal enterocytes that cleave oligosaccharides and disaccharides into absorbable monosaccharides. Acarbose and miglitol competitively inhibit these enzymes, slowing but not completely blocking carbohydrate digestion. Undigested carbohydrates pass into the colon, where bacterial fermentation produces gas and short-chain fatty acids — explaining the class's primary limitation: flatulence, bloating, and diarrhea that affect the majority of patients, particularly at higher doses. Starting at low doses and titrating slowly substantially improves tolerability.

Alpha-glucosidase inhibitors reduce postprandial glucose by 40 to 60 mg/dL and lower glycated hemoglobin by approximately 0.5 to 0.8 percent. Their effect is specifically on postprandial excursions, with minimal effect on fasting glucose. They are taken with the first bite of each meal. Acarbose is minimally absorbed systemically; miglitol has greater systemic absorption but no systemic glucose-lowering effect. Because they act locally, drug interactions are few. Hypoglycemia can occur when alpha-glucosidase inhibitors are combined with insulin or sulfonylureas; it must be treated with pure glucose (glucose tablets) rather than sucrose-containing foods, since sucrose digestion is also inhibited.