A plain-language guide to diabetes medicine classes — metformin, SGLT2 inhibitors, GLP-1 receptor agonists, DPP-4 inhibitors, sulfonylureas and insulin.
Diabetes medicines have changed more in the last decade than in the fifty years before. Some newer classes protect the heart and kidneys as well as lowering sugar. This guide explains each class — how it works, who it tends to suit, common side effects and the safety checks involved. It is educational only: never start, stop or change a medicine without your doctor.
📌 Key Takeaways
• Metformin remains the usual first medicine for type 2 diabetes.
• SGLT2 inhibitors and GLP-1 receptor agonists can protect the heart and kidneys in people at risk.
• Insulin is essential in type 1 diabetes and needed by some people with type 2.
• Side effects are usually manageable with the right choice and monitoring.
• Your doctor chooses medicines around your heart, kidney, weight and budget — not sugar alone.
Understanding diabetes medications, without the jargon.
Modern diabetes pharmacotherapy has been transformed by two drug classes — SGLT2 inhibitors and GLP-1 receptor agonists — that don't just lower glucose but also reduce cardiovascular death, heart failure, and kidney disease progression. Metformin remains the foundation for most T2DM. Sulfonylureas (glimepiride, glipizide) are increasingly de-prioritised due to hypoglycemia and weight gain. Insulin is essential for T1DM and needed for advanced T2DM. All prescriptions require diabetologist evaluation and periodic lab monitoring.
Metformin (Biguanide)
Insulin sensitiser · Oral · First-line for T2DM (ADA/EASD/RSSDI/NICE unanimous)
How it works: Reduces hepatic glucose production, mild peripheral insulin sensitisation, modest GLP-1 elevation. Does NOT cause hypoglycemia when used alone.
Effect on HbA1c: -1.0% to -1.5% typical. Weight-neutral. UKPDS 34 (1998): 32% reduction in diabetes-related endpoints in overweight T2DM.
Common side effects: GI (nausea, diarrhoea, cramping — usually resolves within 2 weeks; XR helps). Vitamin B12 deficiency long-term (~30% at 5+ years — annual B12 check advised).
Contraindications: eGFR <30 (absolute), eGFR 30-45 (reduce dose to 1000 mg/day max), decompensated heart failure, active liver disease, hold before contrast studies (rare lactic acidosis).
Best when: Nearly all T2DM at diagnosis. Also for prediabetes with high risk (BMI ≥35, age <60, prior GDM) — DPP showed 31% risk reduction.
The 68-year story · why metformin remains the anchor of T2DM care
Metformin's active molecule dimethylbiguanide was isolated from Galega officinalis (French lilac / goat's rue), a plant used in medieval Europe for what we now recognise as diabetic symptoms. Synthesised in 1922 by Emil Werner and James Bell, it was clinically introduced by Jean Sterne in France in 1957 under the brand name Glucophage ("glucose eater") [1]. It reached the United States only in 1994 after decades of scepticism driven by the phenformin lactic-acidosis controversy of the 1970s. The landmark UK Prospective Diabetes Study (UKPDS) 34 in 1998 changed everything: in newly-diagnosed overweight T2DM patients, metformin reduced any diabetes-related endpoint by 32%, diabetes-related death by 42%, all-cause mortality by 36%, and myocardial infarction by 39% versus conventional (diet-only) therapy — outcomes unmatched by sulfonylureas or insulin in the same trial [2]. It has been first-line ever since, endorsed by the American Diabetes Association [3], EASD-ADA Consensus [4], NICE NG28 [5], and RSSDI Indian recommendations [6].
Mechanism · far more than "liver glucose lowering"
The primary molecular target of metformin is mitochondrial complex I of the respiratory chain, where it reversibly inhibits ATP production, elevating the cellular AMP:ATP ratio and activating AMP-activated protein kinase (AMPK) [7]. This produces the cascade of metabolic effects: suppression of hepatic gluconeogenesis (the dominant glucose-lowering mechanism, contributing 75% of fasting hyperglycemia reduction), enhanced peripheral insulin sensitivity at muscle and adipose tissue, and modestly increased GLP-1 secretion from L-cells in the gut. Metformin also modifies the gut microbiome, increasing short-chain fatty acid-producing bacteria (Akkermansia muciniphila, Escherichia) — some of the glucose-lowering and even mood-stabilising effects may be mediated via this axis [8]. Because metformin does not stimulate insulin secretion, it does not cause hypoglycemia when used alone — a critical safety advantage over sulfonylureas and insulin.
Beyond glucose · cardiovascular, cancer, longevity signals
UKPDS 34 established metformin's cardiovascular mortality benefit in overweight T2DM — an effect not seen with insulin or sulfonylureas at equivalent glucose lowering [2]. Long-term observational data suggest additional benefits including reduced incidence of several cancers (colorectal, pancreatic, breast, prostate) — hypothesised to reflect AMPK-mediated mTOR inhibition, which slows cellular proliferation. The ongoing TAME (Targeting Aging with Metformin) trial by Nir Barzilai is investigating metformin as a longevity intervention in non-diabetic older adults. Whether these observational signals translate to causal benefit remains debated, but the safety profile of long-term metformin is exceptionally well-established.
Vitamin B12 · the one long-term concern worth screening
Metformin interferes with calcium-dependent B12 absorption in the terminal ileum. Approximately 6% of patients develop biochemical B12 deficiency after 3 years, rising to 30% at 5+ years [11]. Symptoms — peripheral neuropathy, cognitive slowing, glossitis, macrocytic anemia — can mimic diabetic complications and often go undiagnosed. All patients on metformin ≥12 months should have annual serum B12 checked, with methylmalonic acid or holotranscobalamin as second-line if B12 is borderline (150-300 pg/mL). Deficiency is easily corrected with oral cyanocobalamin 1000 mcg daily or monthly IM injections; do not stop metformin over B12 issues alone.
SGLT2 Inhibitors (Empagliflozin, Dapagliflozin, Canagliflozin, Ertugliflozin)
Sodium-glucose co-transporter 2 inhibitor · Oral · Second-line preferred if any CV/renal comorbidity
How it works: Blocks glucose reabsorption in the proximal renal tubule → 60-100 g/day glucose excreted in urine. Also produces natriuresis and modest BP reduction. Osmotic diuresis component drives weight loss.
Effect on HbA1c: -0.6% to -1.0%. Weight loss 2-4 kg. BP reduction 3-5 mmHg. Additional benefits: 38% CV mortality reduction (EMPA-REG), 27% HF hospitalisation reduction (DAPA-HF), kidney protection (DAPA-CKD, EMPA-KIDNEY).
Common side effects: Genital fungal infections (~5-10%, mostly women), UTIs (mild-moderate), volume depletion (especially with diuretics — hold if unwell/dehydrated), rare euglycemic DKA (especially with fasting, illness, low-carb diets — teach ketone awareness).
Contraindications: Type 1 diabetes (higher DKA risk, off-label use only with careful monitoring), eGFR <20-25 for glucose-lowering (some agents extended for CV/renal use to lower eGFR), pregnancy/lactation, recurrent UTI/genital infection
Best when: T2DM with heart failure (any type), CKD (albuminuria or eGFR 25-60), ASCVD, or as second-line to metformin generally per ADA 2025.
The class that rewrote diabetes guidelines
Sodium-glucose co-transporter 2 (SGLT2) inhibitors emerged from a nineteenth-century observation: the French chemist Petersen isolated a compound called phlorizin from apple tree bark in 1835 which caused glucosuria when injected into animals. The clinical translation took 180 years. The FDA approved canagliflozin in 2013 (Invokana, Johnson & Johnson), dapagliflozin in 2014 (Farxiga, AstraZeneca), and empagliflozin in 2014 (, Boehringer/Lilly). Initially these were seen as modest glucose-lowering agents — until the EMPA-REG OUTCOME trial in 2015 produced results that shocked the diabetes field [1]. That single trial, published in the New England Journal of Medicine, showed empagliflozin reduced cardiovascular death by 38%, all-cause mortality by 32%, and heart failure hospitalisation by 35% in T2DM patients with established cardiovascular disease over 3.1 years. It was the first oral diabetes drug ever to show a mortality benefit in a large randomised trial. What followed was a wave of cardiovascular and renal outcome trials that transformed how diabetes, heart failure, and kidney disease are all treated.
Mechanism · so much more than glucose excretion
In healthy adults, the kidney filters ~180 g of glucose daily and reabsorbs ~99% via SGLT2 (90%) and SGLT1 (10%) transporters in the proximal tubule. SGLT2 inhibitors selectively block SGLT2, causing 60-100 g glucose to be excreted in urine daily — an obligate 240-400 kcal daily loss that explains the modest weight reduction (2-4 kg over 6 months). But the glucose-lowering effect is only the beginning. SGLT2 inhibitors produce several parallel effects that likely drive their cardio-renal benefits: natriuresis and osmotic diuresis (reducing preload and afterload — the mechanism behind heart failure benefit), modest blood pressure reduction (3-5 mmHg systolic), reduced intraglomerular hypertension via tubuloglomerular feedback (protects kidney podocytes), improved cardiac energetics through ketone body utilisation as an alternative fuel, and reduced inflammation and oxidative stress [2,3]. These effects are largely independent of glucose lowering, which explains why SGLT2 inhibitors work in patients WITHOUT diabetes.
The cardio-renal outcome trials · a class-wide effect
Every major SGLT2 inhibitor has now been tested in dedicated cardiovascular outcome trials: EMPA-REG OUTCOME (empagliflozin) [1], CANVAS (canagliflozin) [4], DECLARE-TIMI 58 (dapagliflozin) [5], and VERTIS CV (ertugliflozin). Effects on major adverse cardiovascular events are consistent — ~14% relative risk reduction across the class. Heart failure hospitalisation reduction is even more striking (~30%). The DAPA-HF trial in 2019 [6] and EMPEROR-Reduced in 2020 [7] extended this benefit to heart failure with reduced ejection fraction WITHOUT diabetes — the first non-diabetes indication approved. EMPEROR-Preserved in 2021 [8] then extended benefit to heart failure with preserved ejection fraction, the previously untreatable population. Kidney outcomes are equally impressive: CREDENCE (canagliflozin) [9], DAPA-CKD (dapagliflozin) [10], and EMPA-KIDNEY (empagliflozin) [11] each showed 28-39% reductions in kidney disease progression, dialysis initiation, or renal death in CKD — again with or without diabetes.
Clinical indication · when to start
Per ADA 2025 Standards of Care [12] and EASD-ADA Consensus 2022 [13], SGLT2 inhibitors are recommended as second-line after metformin — or as first-line WITHOUT metformin — in T2DM patients with any of: (1) established atherosclerotic cardiovascular disease, (2) heart failure of any type, (3) chronic kidney disease with albuminuria or eGFR 20-60. In these patients, initiate regardless of HbA1c because the benefit is independent of glucose lowering. For T2DM without these comorbidities, SGLT2 inhibitors are still an excellent second-line choice for their weight loss, blood pressure, and hypoglycemia-free profile.
Side effects · what to teach patients
Genital mycotic infections (candidiasis) occur in ~5-10% of women and ~2-5% of men, particularly in the first 3 months. Prevention: daily perineal hygiene, keeping the area dry, prompt treatment with topical antifungals. Rarely warrants discontinuation. Urinary tract infections are 1-2% more common on SGLT2i but are usually mild-moderate. Volume depletion is a real concern in elderly patients or those on loop diuretics — patients should be counselled to hold the SGLT2i (and metformin) during any acute illness with vomiting/diarrhoea or reduced oral intake ("sick day rules"). Euglycemic diabetic ketoacidosis (euDKA) is a rare but serious side effect — DKA occurring at seemingly normal glucose (150-250 mg/dL), triggered by fasting, illness, alcohol binge, ketogenic diet, or surgery. All patients on SGLT2i should be taught ketone awareness (nausea, abdominal pain, rapid breathing) and encouraged to test urine or blood ketones during illness. Fournier's gangrene (perineal necrotising fasciitis) is exceedingly rare (~0.01%) but severe — worth mentioning to men. Modestly increased amputation risk was seen with canagliflozin in CANVAS but not confirmed in later trials — patients with prior lower-limb ulcer or peripheral vascular disease may prefer empagliflozin or dapagliflozin. Bone fracture signal from CANVAS also not replicated in other trials.
GLP-1 Receptor Agonists (Semaglutide, Liraglutide, Tirzepatide GIP/GLP-1)
Incretin mimetics · Injectable (weekly or daily) · Preferred if BMI ≥27 or ASCVD
How it works: Bind GLP-1 receptors (and GIP receptors for tirzepatide). Enhance glucose-dependent insulin secretion, suppress glucagon, delay gastric emptying, reduce appetite via central action. Semaglutide has BOTH SC weekly and oral daily formulations.
Effect on HbA1c & weight: Semaglutide: -1.5% HbA1c, ~15% weight loss (STEP-1). Tirzepatide: -2.0-2.5% HbA1c, ~20-22% weight loss (SURPASS-2, SURMOUNT-1). Cardiovascular benefit proven for semaglutide (SUSTAIN-6, SELECT), liraglutide (LEADER), dulaglutide (REWIND).
Common side effects: GI (nausea 20-40%, vomiting, constipation — mostly first few weeks; titrate slowly), injection-site reactions, rare pancreatitis (~0.2% excess), gallstones with rapid weight loss, delayed gastric emptying can affect other oral meds.
Contraindications: Personal or family history of medullary thyroid carcinoma or MEN-2 (boxed warning, rodent-only signal but caution retained), prior pancreatitis (relative), pregnancy, severe gastroparesis.
Best when: T2DM with obesity (BMI ≥27), established ASCVD, or when significant weight loss is a treatment goal. Also as second injectable before or instead of basal insulin for T2DM.
From lizard venom to blockbuster · the incretin story
The GLP-1 receptor agonist class began with an unlikely observation: in the 1990s, endocrinologist John Eng at the Bronx VA noticed that a peptide called exendin-4 in the saliva of the Gila monster (Heloderma suspectum), a venomous North American lizard, bound to human GLP-1 receptors and produced sustained glucose-lowering effects [1]. This became synthetic exenatide, the first GLP-1 receptor agonist, approved by the FDA in 2005. Liraglutide (Novo Nordisk, ) followed in 2010, semaglutide (, , ) in 2017-2019, and the dual GIP/GLP-1 co-agonist tirzepatide (, Zepbound) in 2022-2023. Along the way, the class has moved from a niche second-line diabetes drug to a cornerstone of both diabetes AND obesity treatment, and — with the SELECT trial in 2023 — established a role in cardiovascular disease prevention independent of diabetes [2].
Mechanism · four levers pulled simultaneously
GLP-1 (glucagon-like peptide-1) is an incretin hormone secreted by intestinal L-cells in response to meals, with a very short native half-life (~2 minutes) due to rapid degradation by DPP-4 enzyme. Long-acting GLP-1 receptor agonists resist DPP-4 breakdown and produce sustained receptor activation, mimicking (and amplifying) the physiological incretin effect. Four key effects work together: (1) Enhanced glucose-dependent insulin secretion — beta cells release more insulin in response to a glucose load, but stop when glucose normalises (no hypoglycemia risk when used alone). (2) Suppression of glucagon secretion from pancreatic alpha cells, particularly postprandial glucagon which drives hepatic gluconeogenesis. (3) Delayed gastric emptying, blunting the postprandial glucose spike and prolonging satiety. (4) Central appetite suppression — GLP-1 receptors in the arcuate nucleus of the hypothalamus and the area postrema reduce food intake and food reward signalling, the mechanism behind the class's weight-loss effect [3]. Tirzepatide adds GIP receptor agonism to the GLP-1 activity, producing additive weight loss and glucose-lowering effects [4].
Efficacy · the numbers by molecule
Semaglutide subcutaneous: STEP-1 in adults with obesity but without diabetes produced 14.9% mean body weight reduction versus 2.4% placebo over 68 weeks [5]. In T2DM (SUSTAIN trials), semaglutide 1 mg weekly reduces HbA1c by 1.5-1.8% with 5-6 kg weight loss. doses up to 2.0 mg add another 0.3% HbA1c reduction. Tirzepatide: SURPASS-2 in T2DM compared tirzepatide 5/10/15 mg weekly vs semaglutide 1 mg weekly — tirzepatide at 15 mg produced 2.3% HbA1c reduction (vs 1.9% for semaglutide) and 11.2 kg weight loss (vs 5.7 kg) [6]. SURMOUNT-1 in adults with obesity showed 20.9% mean weight loss with tirzepatide 15 mg over 72 weeks [7]. Liraglutide: Older but well-established — 1.5% HbA1c reduction with 3-4 kg weight loss. Oral semaglutide () is the only oral GLP-1RA available, achieving efficacy similar to injectable at 14 mg daily but requiring specific administration (empty stomach, ≥30 min before food, sips of water only).
Cardiovascular protection · a class-wide effect
The LEADER trial in 2016 established liraglutide's cardiovascular benefit: 13% relative risk reduction in major adverse cardiovascular events (MACE) and 22% reduction in cardiovascular death over 3.8 years in T2DM at high CV risk [8]. SUSTAIN-6 in 2016 extended this to semaglutide with 26% MACE reduction [9]. REWIND in 2019 showed dulaglutide () reduced MACE by 12% in a broader T2DM population with or without prior CVD [10]. Most transformative was SELECT in 2023: semaglutide 2.4 mg weekly reduced MACE by 20% in adults with obesity and prior cardiovascular disease but WITHOUT diabetes — the first time an anti-obesity medication showed cardiovascular event reduction independent of diabetes [2]. Tirzepatide's dedicated CV outcome trial (SURPASS-CVOT) is expected to report in 2026.
Side effects · manage the GI, respect the rare
Nausea affects 20-40% of patients in the first weeks, typically improving as titration proceeds. Vomiting occurs in 5-10%, constipation in 10-15%. Slow titration (4-week increments), eating smaller portions, avoiding high-fat meals, and staying well-hydrated dramatically improve tolerability. About 5-10% of patients discontinue due to GI intolerance. Injection-site reactions — mild redness or itching — occur in 3-5% and resolve with site rotation. Pancreatitis has a small excess risk (~0.2% versus placebo) — history of pancreatitis is a relative contraindication. Gallstones occur more commonly with GLP-1 RAs, particularly with rapid weight loss (0.5-2% incidence over 1-2 years) — screen for symptoms and offer ursodeoxycholic acid prophylaxis in high-risk patients. Delayed gastric emptying can affect absorption of other oral medications and creates increased perioperative aspiration risk — new ASA 2023 guidance recommends holding GLP-1 RAs for 1 week before elective surgery/anaesthesia [11]. Medullary thyroid carcinoma risk carries an FDA boxed warning based on rodent studies, though human epidemiological data over 15+ years have not confirmed this signal — but personal or family history of medullary thyroid cancer or MEN2 remains a firm contraindication. Diabetic retinopathy progression was seen in SUSTAIN-6 with semaglutide (relative risk 1.76 for retinopathy complications) — likely reflecting rapid glucose normalisation rather than direct drug effect; screen retinopathy before starting in poorly-controlled diabetes.
DPP-4 Inhibitors (Sitagliptin, Vildagliptin, Linagliptin, Teneligliptin)
Enhance endogenous incretins · Oral · Well-tolerated, modest efficacy · Popular in India
How it works: Inhibit DPP-4 enzyme that degrades endogenous GLP-1 and GIP. Prolongs incretin effect. Weight-neutral, no hypoglycemia alone.
Effect on HbA1c: -0.5% to -0.8%. Weight-neutral. CV-neutral in outcome trials (TECOS sitagliptin, SAVOR-TIMI saxagliptin — though saxagliptin has heart failure signal).
Best when: Elderly T2DM, renal impairment (linagliptin needs no dose adjustment), Ramadan/religious fasting, patients preferring oral-only regimens
Insulin (Basal-Bolus, MDI, or Insulin Pump)
Exogenous hormone replacement · Injectable · Essential for T1DM, needed in advanced T2DM
How it works: Directly replaces endogenous insulin. Modern analogues offer smoother action profiles vs older human insulin. Basal (glargine U100/U300, degludec) provides 24-hr background; bolus (aspart, lispro, ultra-rapid aspart) covers meals.
Typical regimens: T1DM: MDI (basal + 3× bolus with carb counting) OR insulin pump (basal infusion + boluses). T2DM: basal only first (0.1-0.2 units/kg), add bolus if postprandial spikes persist. Modern hybrid closed-loop for T1DM (T:slim X2, Omnipod 5) increasingly available.
Effect on HbA1c: -1.5% to -3.0% (largest of any class, fastest onset). Enables tight control even at very high starting HbA1c.
Common side effects: Hypoglycemia (dose-dependent, most common serious side effect), weight gain (2-6 kg typical), injection-site lipohypertrophy (rotate sites), rare allergic reactions.
Best when: All T1DM (always). T2DM: HbA1c >10% at diagnosis (glucotoxicity — often temporary), persistent HbA1c >8.5% on triple oral therapy, pregnancy, hospitalisation, steroid use, acute illness.
Sulfonylureas (Glimepiride, Gliclazide, Glipizide) — De-Prioritised
Insulin secretagogues · Oral · Older class · Reserved for cost-limited settings only
How it works: Bind pancreatic beta-cell K-ATP channels, forcing insulin secretion regardless of glucose level (hence hypoglycemia risk).
Effect on HbA1c: -1.0% to -1.5% initially. Effect declines over 3-5 years as beta cells fail (secondary sulfonylurea failure).
Concerns: Hypoglycemia (especially glimepiride, glibenclamide — avoid), weight gain 2-5 kg, beta-cell exhaustion accelerating T2DM progression, possibly increased CV mortality vs metformin (UKPDS observational). NOT recommended in impaired hypoglycemia awareness.
Best when: Only when SGLT2i/GLP-1RA/DPP-4i cannot be afforded — otherwise increasingly replaced per ADA 2025 (relegated to fourth-line)
⚠ Important safety notes: Never start, stop, or change diabetes medications without diabetologist guidance — hypoglycemia (glucose <70 mg/dL) can be dangerous, and rapid glucose reduction can transiently worsen retinopathy. Modern practice avoids sulfonylureas and human basal insulin when SGLT2i/GLP-1RA/analog insulins are affordable. Watch for euglycemic DKA on SGLT2i (especially during illness, fasting, ketogenic diets — teach ketone testing). SGLT2i and GLP-1RA can be combined for additive HbA1c and complementary CV/renal benefits (VERTIS CV, AMPLITUDE-O).
Currently on medications you're unsure about?
Sulfonylurea causing hypos? Not on an SGLT2i despite having CKD/heart failure?
