Non-Insulin Glucose-Lowering Agents

Metformin: First-Line Therapy

Metformin, a biguanide, remains first-line pharmacologic therapy for most people with type 2 diabetes because of its glycemic efficacy, weight neutrality to modest weight loss, low hypoglycemia risk when used alone, favorable cardiovascular safety record, and low cost.

• Mechanism: primarily reduces hepatic glucose production, with a modest additional improvement in peripheral insulin sensitivity.
• GI effects: the most common adverse effect, occurring in roughly 20-30% of new users (nausea, diarrhea, abdominal discomfort). These effects typically diminish over days to weeks and are mitigated by starting at a low dose, titrating gradually, taking the dose with food, and/or using an extended-release formulation.
• Vitamin B12 deficiency: long-term metformin use interferes with the calcium-dependent uptake of the intrinsic-factor-B12 complex in the ileum. A clinically meaningful biochemical deficiency develops in roughly 5-10% of long-term users, more often at higher doses. Periodic B12 monitoring is recommended, particularly at doses of 1,500 mg/day or greater, with prolonged use, or if anemia or peripheral neuropathy symptoms develop.
• Lactic acidosis: rare but serious; risk rises with the degree of renal impairment because metformin is cleared renally, essentially unchanged.
• ★ Renal dosing rule: metformin is contraindicated at an eGFR below 30 mL/min/1.73 m². Initiation is not recommended for an eGFR of 30-45 mL/min/1.73 m². Around iodinated contrast administration, metformin should be held and renal function rechecked about 48 hours later before resuming in anyone with an eGFR of 30-60, hepatic impairment, alcohol use disorder, or heart failure; at an eGFR of 30 or above with no acute kidney injury, metformin can generally be continued through contrast administration.

Non-Insulin Agent Comparison

★ GLP-1 Receptor Agonists and Dual GIP/GLP-1 Agents

These incretin-based therapies produce substantial weight loss — greatest with the dual GIP/GLP-1 agonist tirzepatide — while improving glycemic control, and several GLP-1 receptor agonists with dedicated cardiovascular outcome trials reduce major adverse cardiovascular events in people with established ASCVD. Gastrointestinal effects (nausea, vomiting, diarrhea, constipation) are dose-dependent and common; doses are escalated slowly over weeks specifically to improve tolerability, and symptoms typically improve over time. Because these agents slow gastric emptying, they warrant caution — and are a relative contraindication — in people with a history of severe gastroparesis. They should not be initiated in anyone with a history of pancreatitis and should be discontinued if pancreatitis is suspected during treatment.

All GLP-1 receptor agonists and dual GIP/GLP-1 agonists carry an FDA boxed warning against use in people with a personal or family history of medullary thyroid carcinoma (MTC) or Multiple Endocrine Neoplasia syndrome type 2 (MEN2). This warning is based on thyroid C-cell tumors observed in rodent studies; a causal link in humans has not been established, but the contraindication applies regardless.

★ SGLT2 Inhibitors

SGLT2 inhibitors lower glucose independent of insulin by blocking glucose reabsorption in the proximal renal tubule, producing glucosuria. Beyond glycemic control, they reduce heart-failure hospitalization and slow progression of chronic kidney disease — benefits recognized independent of baseline A1C or metformin use. Their leading safety concerns are genital mycotic infections (the most common adverse effect, mitigated with hygiene counseling) and euglycemic diabetic ketoacidosis (euglycemic DKA) — DKA occurring with a near-normal or only mildly elevated glucose. Because SGLT2 inhibitor-associated DKA commonly presents with glucose under 200 mg/dL, glucose level alone cannot rule out DKA; ketones must be checked whenever a person on an SGLT2 inhibitor has DKA symptoms such as nausea, vomiting, abdominal pain, or malaise. Recognized risk factors include prolonged fasting, very-low-carbohydrate diets, dehydration, excess alcohol use, acute illness, surgery, and insulin under-dosing or omission — the CDCES teaches patients to hold SGLT2 inhibitors before scheduled surgery and during acute illness with reduced oral intake.

DPP-4 Inhibitors, Sulfonylureas, and TZDs

DPP-4 inhibitors are weight-neutral with a low hypoglycemia risk when used alone, but their cardiovascular safety is not uniform across the class: saxagliptin and alogliptin increase the risk of heart-failure hospitalization, while sitagliptin and linagliptin have not shown this signal in dedicated cardiovascular outcome trials. Sulfonylureas carry the highest hypoglycemia risk of any oral agent because they stimulate insulin secretion independent of glucose level, plus consistent weight gain; glyburide is the highest-risk agent in the class, particularly in older adults or renal impairment. Pioglitazone, the primary TZD in current use, improves peripheral insulin sensitivity but causes fluid retention that is contraindicated in NYHA class III-IV heart failure, along with an increased fracture risk and weight gain.

Two testable distinctions: "weight neutral" (DPP-4 inhibitors) is not the same as "weight loss" (GLP-1 RA, dual GIP/GLP-1, SGLT2i), and metformin's renal cutoff is a hard contraindication at eGFR <30, while the 30-45 range is only a "do not initiate" caution rather than an absolute contraindication for someone already stable on the drug.