Which insulins are considered slow‑acting (basal) insulin?

Slow-Acting (Basal) Insulins

Slow-acting insulins—also called basal insulins—include NPH insulin, long-acting insulin analogs (insulin glargine, insulin detemir, and insulin degludec), and continuous subcutaneous infusion of rapid-acting insulin via pump. These formulations provide sustained background insulin coverage to suppress hepatic glucose production between meals and overnight 1.

Long-Acting Insulin Analogs (Preferred Basal Insulins)

First-Generation Long-Acting Analogs

• Insulin glargine (Lantus, Basaglar) has an onset of approximately 1 hour, a peakless profile, and duration of action up to 24 hours, allowing once-daily dosing 2, 3.
• Insulin detemir (Levemir) has a duration of action less than 24 hours and may require twice-daily dosing in some patients 1, 4.

Second-Generation Long-Acting Analogs

• Insulin glargine U-300 (Toujeo) provides longer duration of action than U-100 glargine with more stable 24-hour coverage and reduced nocturnal hypoglycemia risk 5.
• Insulin degludec (Tresiba) has ultra-long duration exceeding 24 hours with highly consistent day-to-day absorption and lower nocturnal hypoglycemia risk compared to glargine U-100 6, 7.

Intermediate-Acting Insulin

• NPH insulin is an intermediate-acting insulin with a pronounced peak effect and duration of 12–18 hours, typically requiring twice-daily dosing 1, 8.
• NPH has a higher risk of nocturnal hypoglycemia compared to long-acting analogs due to its peak action profile 2, 3.
• NPH no longer has a place in the treatment of type 1 diabetes because its early peak effects and relatively short duration result in nocturnal hypoglycemia risk and fasting hyperglycemia after evening injection 8.
• NPH remains the most cost-effective basal insulin option (approximately $25–$165 per vial) when hypoglycemia risk is not elevated 9.

Continuous Subcutaneous Insulin Infusion (CSII)

• Insulin pump therapy using rapid-acting insulin analogs provides basal insulin through continuous delivery and is considered the "gold standard" for basal insulin replacement due to better reproducibility of subcutaneous absorption 8.
• CSII may be indicated in subsets of type 1 diabetes patients with insulin "supersensitivity," low-dose insulin needs, or variable subcutaneous absorption of long-acting analogs to minimize glucose variability and reduce hypoglycemia 8.

Clinical Advantages of Long-Acting Analogs Over NPH

• Long-acting basal analogs are nearly peakless and reduce the risk of hypoglycemia (especially nocturnal), blood glucose variability, and lower A1C with similar or less hypoglycemia compared to NPH 1, 8.
• Insulin glargine demonstrates a steady time-action profile over 24 hours without a pronounced peak, producing lower fasting glucose levels and consistent reduction in nocturnal hypoglycemia compared to NPH 3.
• Second-generation analogs (glargine U-300 and degludec) provide more stable glycemic control with reduced hypoglycemia risk compared to first-generation analogs 7.

Key Distinction: Basal vs. Prandial Insulin

• Basal insulin (slow-acting) restrains hepatic glucose production between meals and overnight, controlling fasting and between-meal glucose levels 1, 10.
• Prandial insulin (rapid-acting analogs like lispro, aspart, glulisine) has quick onset (0.25–0.5 hours), peak at 1–3 hours, and duration of 3–5 hours to cover postprandial glucose excursions 1, 10.
• In type 1 diabetes, basal insulin typically comprises 40–60% of total daily insulin dose, with the remainder as prandial insulin 1, 10.