Diabetes medicine

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Diabetes Medicines: Mechanisms & Side Effects (In-Depth)

Below is a class-by-class breakdown of all major antidiabetic drug classes, their mechanisms of action, and clinically important side effects, drawn from Goodman & Gilman's Pharmacological Basis of Therapeutics (16th ed.) and Harrison's Principles of Internal Medicine (22nd ed., 2025).

1. Insulin

Types & Mechanisms

Insulin acts by binding to insulin receptors on liver, muscle, and adipose tissue, promoting glucose uptake and suppressing hepatic gluconeogenesis. Structural modifications to human insulin alter its absorption kinetics:

Type	Examples	Onset	Duration
Rapid-acting analogues	Lispro, Aspart, Glulisine	~15 min	3-5 h
Short-acting (Regular)	Regular insulin	30-45 min	6-8 h
Intermediate-acting	NPH (insulin isophane)	1-2 h	12-18 h
Long-acting analogues	Glargine, Detemir, Degludec	1-2 h	~24 h (flat profile)

Lispro/Aspart/Glulisine: Amino acid substitutions prevent hexamer formation → faster subcutaneous absorption → inject just before meals
Glargine: Acidic pH stabilizes hexamers subcutaneously → prolonged flat release with minimal peak; cannot be mixed with short-acting insulins due to pH mismatch
NPH: Zinc-protamine complex → delayed dissolution; cloudy appearance

Side Effects

Hypoglycemia (most common and serious) - risk is 50% higher variance in absorption between doses
Weight gain
Injection site reactions (lipodystrophy, infections with insulin pumps)
Hypokalemia (insulin drives K+ into cells)

2. Metformin (Biguanide)

Mechanism

Metformin primarily inhibits hepatic gluconeogenesis (activates AMPK → suppresses gluconeogenic enzymes). It also reduces intestinal glucose absorption and modestly improves peripheral insulin sensitivity. It does NOT stimulate insulin secretion.

Side Effects

GI effects (most common): diarrhea, nausea, anorexia, metallic taste - occur in up to 30%; extended-release formulation reduces this significantly
Lactic acidosis (rare but serious) - risk increases with renal impairment, contrast agents, or hepatic disease; hold before IV contrast
Does NOT cause hypoglycemia when used alone
Vitamin B12 deficiency with chronic use

Source: Harrison's Principles of Internal Medicine 22E, p. 3255

3. Sulfonylureas

Mechanism

Bind to SUR1 subunits of ATP-sensitive K+ channels on pancreatic beta cells → channel closure → membrane depolarization → Ca2+ influx → insulin release. Effect is glucose-independent.

Examples: Glipizide, Glyburide (Glibenclamide), Glimepiride (2nd generation)

Side Effects

Hypoglycemia - most important; especially with glyburide (longer-acting); elderly at high risk
Weight gain (1-2 kg average)
Rare: hematologic effects (agranulocytosis), hepatotoxicity, photosensitivity
Avoid in renal impairment (especially glyburide)

4. Thiazolidinediones (TZDs / Glitazones)

Mechanism

Activate PPARγ (peroxisome proliferator-activated receptor gamma) in adipose tissue and liver → increased expression of insulin-sensitizing proteins → improved glucose uptake in muscle and adipose. Require endogenous insulin to work; NOT effective in Type 1 DM.

Examples: Pioglitazone, Rosiglitazone

Side Effects

Weight gain (2-4 kg in first year) + peripheral edema (fluid retention) - dose-related
Heart failure risk: up to 2x increased incidence - contraindicated in moderate-to-severe heart failure
Macular edema
Increased fracture risk (especially distal limb fractures in women)
Pioglitazone: reduces triglycerides ~10-15%, raises HDL - favorable lipid profile
Rosiglitazone: raises LDL cholesterol; historically associated with increased CV risk (though later evidence was controversial)

Source: Goodman & Gilman's, p. 1056

5. GLP-1 Receptor Agonists (GLP-1 RAs / "Incretin Mimetics")

Mechanism

Mimic glucagon-like peptide-1 (GLP-1), an incretin hormone released from intestinal L-cells after meals. Actions:

Stimulate glucose-dependent insulin secretion (only when glucose is elevated)
Suppress glucagon secretion
Slow gastric emptying
Reduce appetite (central CNS effect)
Promote weight loss

Examples: Semaglutide (weekly SC or daily oral), Liraglutide (daily SC), Dulaglutide (weekly), Exenatide, Lixisenatide

Side Effects

GI: nausea, vomiting, diarrhea - most common, especially on initiation; usually transient
Hypoglycemia rare when used alone; risk increases when combined with sulfonylureas
Pancreatitis - possible association; use with caution in history of pancreatitis
Gallbladder disease (gallstones) - increased risk with liraglutide and semaglutide
Thyroid C-cell tumors in rodents → contraindicated in medullary thyroid carcinoma or MEN2
Exenatide: avoid in renal failure (CrCl <30 mL/min)
Cardiovascular benefit: LEADER (liraglutide) and SUSTAIN-6 (semaglutide) trials showed reduced CV mortality, MI, stroke - now approved for HFREF

Source: Goodman & Gilman's; Fuster and Hurst's The Heart, 15th ed.

6. DPP-4 Inhibitors ("Gliptins")

Mechanism

Inhibit dipeptidyl peptidase-4 (DPP-4), the enzyme that normally degrades GLP-1 and GIP within minutes. DPP-4 inhibition raises active GLP-1 from ~10% to ~100% of circulating levels → increased insulin secretion, reduced glucagon, improved fasting and postprandial glucose.

Examples: Sitagliptin, Saxagliptin, Linagliptin, Alogliptin, Vildagliptin (EU)

Side Effects

Generally very well tolerated - "weight-neutral" class
Nasopharyngitis, upper respiratory infections (common)
Urinary tract infections
Pancreatitis (rare; post-marketing reports - same caution as GLP-1 RAs)
Heart failure hospitalizations with saxagliptin (SAVOR-TIMI trial) - avoid in heart failure
Dose reduction needed in renal impairment (except linagliptin, which is hepatically cleared)
Do NOT cause hypoglycemia when used alone (glucose-dependent mechanism)

Source: Goodman & Gilman's, p. 2991-2995

7. SGLT2 Inhibitors ("Gliflozins")

Mechanism

Block sodium-glucose co-transporter 2 (SGLT2) in the proximal renal tubule → prevent glucose reabsorption → urinary glucose excretion (~70-80 g/day). Glucose lowering is insulin-independent. Additional effects: natriuresis, reduced blood pressure, inhibition of NHE (sodium-hydrogen exchanger) in the heart, shift from glucose to fatty acid/ketone utilization for ATP production.

Examples: Empagliflozin, Dapagliflozin, Canagliflozin, Ertugliflozin

Side Effects

Genital mycotic infections (most common) - due to glucosuria creating a favorable environment for fungal overgrowth
UTIs
Diabetic ketoacidosis (DKA) - even with near-normal glucose ("euglycemic DKA") - especially in Type 1 or perioperative settings
Volume depletion / hypotension - particularly with diuretics or elderly
Fournier's gangrene (necrotizing fasciitis of perineum) - rare but serious
Amputations - raised risk with canagliflozin specifically
Reduced eGFR on initiation (transient)
Do NOT cause hypoglycemia (insulin-independent)

Major Benefits Beyond Glucose

Heart failure: empagliflozin and dapagliflozin reduce mortality and hospitalizations in both HFrEF and HFpEF (now approved regardless of diabetes status)
Chronic kidney disease: slow CKD progression, reduce proteinuria

Source: Goodman & Gilman's; Katzung's Basic & Clinical Pharmacology, 16th ed.

8. Alpha-Glucosidase Inhibitors

Mechanism

Competitively inhibit intestinal alpha-glucosidases → delay carbohydrate digestion and glucose absorption → blunt postprandial hyperglycemia.

Examples: Acarbose, Miglitol

Side Effects

GI: flatulence, bloating, diarrhea, abdominal cramps (very common - due to fermentation of undigested carbohydrates in the colon)
No systemic absorption → minimal systemic toxicity
Do not cause hypoglycemia alone; if hypoglycemia occurs (from combined therapy), treat with glucose tablets, NOT sucrose (which requires the blocked enzyme)

9. Amylin Analogue (Pramlintide)

Mechanism

Analogue of amylin, co-secreted with insulin from beta cells. Slows gastric emptying, suppresses post-meal glucagon, reduces food intake. Used as an adjunct to mealtime insulin in Type 1 and Type 2 DM.

Side Effects

Nausea (most common, especially on initiation)
Hypoglycemia (when used with insulin - insulin dose often must be reduced)
Injected before meals (SC); cannot be mixed with insulin

Quick Comparison Table

Drug Class	Primary Mechanism	Hypoglycemia Risk	Weight Effect	Key Concern
Insulin	Direct glucose uptake	High	Gain	Hypoglycemia, injection site
Metformin	↓ Hepatic gluconeogenesis	None	Neutral/loss	Lactic acidosis (renal impairment)
Sulfonylureas	Beta cell K+ channel closure → insulin release	High	Gain	Hypoglycemia (elderly)
TZDs	PPARγ activation → insulin sensitization	None	Gain + edema	Heart failure, fractures
GLP-1 RAs	Glucose-dependent insulin, ↓ glucagon, ↓ appetite	Low (alone)	Loss	Nausea, pancreatitis, thyroid
DPP-4 inhibitors	↑ Active GLP-1/GIP	None	Neutral	Pancreatitis, UTI, saxagliptin/HF
SGLT2 inhibitors	Urinary glucose excretion	None	Loss	Genital infections, euglycemic DKA
Alpha-glucosidase inhibitors	↓ Carbohydrate absorption	None	Neutral	Flatulence/GI (very common)
Pramlintide	↓ Gastric emptying, ↓ glucagon	Low (with insulin)	Loss	Nausea

Sources: Goodman & Gilman's Pharmacological Basis of Therapeutics | Harrison's Principles of Internal Medicine, 22E (2025) | Katzung's Basic and Clinical Pharmacology, 16th ed. | Fuster and Hurst's The Heart, 15th ed.

For the most current cardiovascular outcomes and renal benefits of newer agents (SGLT2i, GLP-1 RAs), the ADA Standards of Care 2025 and recent trial data continue to refine prescribing priorities.

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