Insulin — Types, Pharmacokinetics & Delivery Systems

Why Insulin Pharmacokinetics Is High-Yield and Patient-Safety-Critical

Every person with type 1 diabetes, and a large and growing share of people with type 2 diabetes, depends on exogenous insulin to survive or reach glycemic targets. The CDCES translates insulin action profiles into concrete guidance — when to inject relative to a meal, how to interpret a low reading on a monitoring log, how to counsel about a missed or delayed dose — so a wrong onset, peak, or duration fact does not stay theoretical; it changes real dosing decisions and can cause hypoglycemia or prolonged hyperglycemia. This section presents the verified time-action profile for every insulin category tested on the exam, along with available concentrations and modern delivery systems.

Insulin Time-Action Profiles

Rapid-acting analogs are dosed within about 15 minutes before, or immediately after, a meal because their onset closely mimics the body's natural mealtime insulin release. They are the standard choice for mealtime (bolus) dosing, correction doses, and are the only insulins used in pumps and hybrid closed-loop systems. Ultra-rapid formulations — faster-acting insulin aspart ("Fiasp") and insulin lispro-aabc ("Lyumjev") — shave several additional minutes off onset by adding excipients that accelerate absorption from the injection site.

Regular (short-acting) insulin should be injected roughly 30 minutes before eating so its slower onset lines up with meal absorption; injecting it at mealtime, the way rapid-acting analogs are dosed, produces a mismatch and a post-meal glucose spike. Outside of prandial dosing, regular insulin is the only insulin approved for intravenous administration and is therefore the formulation used to treat diabetic ketoacidosis (DKA) and hyperosmolar hyperglycemic state (HHS) in the hospital.

NPH is a cloudy, protamine-bound suspension that must be gently rolled or inverted — never shaken — to resuspend evenly before every dose; inconsistent resuspension is a common, correctable source of erratic glucose control. NPH is the only commonly used basal insulin with a clinically pronounced peak. That peak is both its therapeutic rationale (a deliberate "hump" of activity timed to cover a specific meal) and its major safety liability: mistimed NPH peaks are a classic, testable cause of nocturnal or mid-afternoon hypoglycemia.

Long-acting basal analogs (glargine, detemir) are engineered to release at a relatively constant rate with no pronounced peak, which lowers hypoglycemia risk compared with NPH. Glargine at standard U-100 concentration is typically dosed once daily. Detemir's duration is dose-dependent and can fall short of 24 hours at lower doses, sometimes requiring twice-daily dosing to maintain full-day coverage.

Degludec is the longest-acting basal insulin available, with a duration exceeding 42 hours and an exceptionally flat action profile. Because its activity persists so long, a delayed or occasionally missed dose is more forgiving than with other basal insulins, but switching to or from degludec requires attention to the overlapping tail of activity from the prior insulin.

Premixed insulins (for example, 70/30 NPH/regular, or 75/25 and 50/50 lispro-protamine/lispro) combine a fixed ratio of an intermediate- or long-acting component with a rapid- or short-acting component in one injection. They reduce the number of daily injections, but the two components cannot be titrated independently — a limitation when meal size or activity varies day to day.

Insulin Concentrations

Most insulin is manufactured at U-100 (100 units/mL), but several concentrated products exist for people with higher insulin requirements or specific pharmacokinetic goals:

• U-200: insulin degludec (Tresiba) and insulin lispro (Humalog KwikPen) — twice the concentration of U-100, delivered in half the injection volume for an equivalent dose.
• U-300: insulin glargine (Toujeo) — the higher concentration forms a smaller, more compact subcutaneous depot that releases even more gradually than U-100 glargine, producing a flatter, more prolonged action profile.
• U-500: regular human insulin (Humulin R U-500) — five times more concentrated than U-100 regular insulin, reserved for people with severe insulin resistance requiring more than 200 units per day. Because of its concentrated depot, U-500 behaves with both prandial and basal characteristics, showing a slower onset and longer duration than U-100 regular. It is dispensed only in a dedicated prefilled pen dosed in 5-unit increments (up to 300 units per injection). U-500 must never be drawn up with a standard U-100 syringe, and it is not approved for use in standard insulin pumps — both are recognized, high-severity medication-error risks.

Delivery Systems

• Vial and syringe — lowest cost, but manual dose drawing is a documented source of dosing errors, especially with concentrated insulins.
• Insulin pens (prefilled disposable or reusable) — improve dosing accuracy and convenience over vial-and-syringe technique.
• Insulin pumps (CSII) — deliver only rapid-acting insulin through a catheter via a programmable basal rate plus patient-initiated meal and correction boluses.
• Automated insulin delivery (AID) / hybrid closed-loop systems — pair an insulin pump with a continuous glucose monitor (CGM) and a dosing algorithm that automatically raises, lowers, or suspends basal insulin delivery in response to real-time sensor glucose trends; many current systems also deliver automatic correction boluses. These systems are called "hybrid" because the user still must manually announce and bolus for meals. Across clinical trials, AID/hybrid closed-loop use increases Time-in-Range and reduces both hypoglycemia and hyperglycemia burden compared with standard pump therapy or multiple daily injections.

Two testable distinctions: "peakless" does not mean "no glucose-lowering effect" — glargine, detemir, and degludec still lower glucose steadily; they simply lack NPH's pronounced spike. And U-500 is regular human insulin at a higher concentration, not a distinct analog — its unique prandial-plus-basal behavior comes entirely from the concentrated depot.