Sulfonylureas, Thiazolidinediones & the Rest
Before the incretins and flozins, these were the drugs that filled the gap after metformin — and they're still widely used because they're effective and cheap. But unlike the newer classes, the sulfonylureas can cause hypos and weight gain. Knowing exactly why tells you when they help and when to be careful.
The diabetes toolkit has a newer wing — metformin, incretins, flozins — that tends to avoid hypos and weight gain. This article covers the older wing: drugs that work by squeezing more insulin out of the pancreas, or by making tissues more sensitive in a different way. They're older, cheaper, and still very much in use, but they behave differently from the modern favourites, and the differences are exactly what you need to remember. The headline drugs are the sulfonylureas, and the single most important thing to know about them is that, because they force insulin out, they carry the two risks the newer drugs avoid: hypoglycaemia and weight gain.
Sulfonylureas — the insulin squeezers
They force the pancreas to release insulin — regardless of glucose level. Sulfonylureas (gliclazide, glimepiride, glipizide, and the older glibenclamide) work by directly stimulating the beta cells of the pancreas to release insulin. Notice the crucial contrast with GLP-1 drugs: sulfonylureas force insulin out whether or not blood glucose is high — the release is NOT glucose-dependent. That single fact explains their two characteristic downsides. Because insulin can be pushed out even when glucose is already normal or low, sulfonylureas can cause hypoglycaemia — the main serious risk, and one that can be severe and prolonged, especially in the elderly or those with kidney impairment (glibenclamide is the worst offender and is best avoided in older patients). And because insulin is an anabolic, storage hormone, the extra insulin promotes weight gain. Against those drawbacks, they are effective, familiar and inexpensive, which keeps them in wide use — but they only work if the pancreas still has insulin to give, so they are useless in type 1 diabetes. A related, shorter-acting group, the meglitinides ('glinides', like repaglinide), work the same way but faster and briefer, taken with meals.
Thiazolidinediones and the others
The thiazolidinediones ('glitazones', mainly pioglitazone) take a different approach: like metformin they are insulin SENSITISERS, making muscle and fat respond better to insulin, so they don't cause hypoglycaemia on their own. But they carry their own baggage: they cause weight gain and, importantly, fluid retention — which means they must be avoided in heart failure, where extra fluid is dangerous. They can also slightly raise fracture risk. A couple of older or niche agents round out the picture: acarbose (an alpha-glucosidase inhibitor) slows the digestion of carbohydrate in the gut to blunt the after-meal glucose spike, but causes bloating and flatulence and is little used; and pramlintide (an amylin analogue) is a rare adjunct. For the exam and the ward, though, the practical map is simple: metformin first; then choose the next drug by the patient — a flozin or GLP-1 agonist if the heart or kidneys need protecting or weight loss is wanted, and a sulfonylurea when cost is the priority and hypoglycaemia can be watched for.
- Sulfonylureas (gliclazide, glimepiride) force insulin out — NOT glucose-dependent.
- So they CAN cause hypoglycaemia and weight gain — the two risks newer drugs avoid.
- Useless in type 1 (need a working pancreas); glibenclamide worst for hypos in the elderly.
- Pioglitazone (a glitazone) is a sensitiser — no hypos, but fluid retention (avoid in heart failure) + weight gain.
- Acarbose blunts post-meal spikes (bloating/flatulence); overall map: metformin first, then tailor.
This whole article hangs on one contrast you've now met three times: glucose-dependent versus glucose-independent insulin release. GLP-1 drugs raise insulin only when glucose is high, so they're self-limiting and don't cause hypos. Sulfonylureas raise insulin no matter what the glucose is doing, so they can overshoot into a hypo and, by delivering more of the storage hormone insulin, cause weight gain. That single axis — does the drug's insulin release switch off when glucose is normal? — cleanly predicts which diabetes drugs are 'safe alone' and which need watching. Learn to ask that question of any new agent and you'll rarely be surprised by its side effects.
- Giving a sulfonylurea to a type 1 patient — the pancreas can't respond.
- Using glibenclamide in an elderly or renally-impaired patient — severe, prolonged hypos.
- Prescribing pioglitazone in heart failure — fluid retention worsens it.
- Forgetting that sulfonylureas cause weight gain — a drawback in an overweight patient.
Why can sulfonylureas cause hypoglycaemia while GLP-1 agonists usually don't?
- Sulfonylureas force out insulin (glucose-independent) → can cause hypos and weight gain; useless in type 1.
- Prefer gliclazide over glibenclamide (safer for hypos, esp. elderly/renal).
- Pioglitazone is a sensitiser (no hypos) but causes fluid retention (avoid in heart failure) + weight gain.
- Overall: metformin first, then tailor by the patient's heart, kidney, weight and cost.
- Katzung BG. Basic & Clinical Pharmacology — Pancreatic Hormones & Antidiabetic Drugs (secretagogues, TZDs).
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Oral antidiabetic agents.
- ADA — Standards of Care in Diabetes (pharmacologic therapy).
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Sulfonylureas & thiazolidinediones.
