Prescribing insulin starts with matching the right insulin type and dose to the patient’s needs, then adjusting systematically based on blood glucose readings. The process differs between type 1 and type 2 diabetes, but the core framework involves selecting a basal insulin, calculating a starting dose, adding mealtime insulin when needed, and building in a plan for dose titration and hypoglycemia management.
Choosing the Right Insulin Type
Insulins fall into categories based on how quickly they act and how long they last. Selecting the right combination depends on whether you’re covering baseline glucose production, post-meal spikes, or both.
- Rapid-acting (lispro, aspart, glulisine): Onset in 12 to 30 minutes, peaks at 1 to 3 hours, lasts 2 to 6 hours. Used before meals to cover carbohydrate intake.
- Ultra-rapid (faster aspart): Onset in about 5 minutes, peaks at 30 minutes, lasts 3 to 5 hours. Useful when tighter post-meal control is needed.
- Short-acting (regular insulin): Onset in 30 to 60 minutes, peaks at 2 to 4 hours, lasts 5 to 8 hours. Often used in hospital settings or mixed regimens.
- Intermediate-acting (NPH): Onset in 2 to 4 hours, peaks at 4 to 10 hours, lasts 8 to 16 hours. Can serve as basal coverage but has a pronounced peak that increases hypoglycemia risk.
- Long-acting (glargine, detemir): Onset in 1 to 4 hours, minimal or no peak, lasts 20 to 24 hours. The standard choice for once-daily basal coverage.
- Ultra-long-acting (degludec, glargine U-300): Duration extends to 36 to 42 hours. Provides the flattest basal profile and the most flexible dosing window.
For most patients starting insulin, a long-acting or ultra-long-acting basal insulin is the first prescription. It controls fasting glucose with less hypoglycemia risk than NPH. Mealtime rapid-acting insulin gets added later if post-meal glucose remains elevated despite optimized basal dosing.
Starting Doses for Type 2 Diabetes
The ADA recommends initiating basal insulin at 10 units per day or 0.1 to 0.2 units/kg/day. This applies when A1C remains uncontrolled after three or more months of combination oral therapy, when A1C exceeds 10%, when blood glucose is above 300 mg/dL, or when the patient has symptoms of hyperglycemia. Starting conservatively and titrating upward is safer than beginning with a high dose.
Most patients with type 2 diabetes begin with basal insulin alone, taken once daily (typically at bedtime or in the morning, depending on the formulation). Oral medications like metformin are generally continued alongside insulin. The goal at this stage is to bring fasting blood glucose into range, which then lowers overall A1C.
Starting Doses for Type 1 Diabetes
In type 1 diabetes, the total daily dose (TDD) of insulin is typically calculated on a weight basis, commonly starting around 0.4 to 0.5 units/kg/day for newly diagnosed adults. This total is then split between basal and mealtime (bolus) insulin. Basal insulin makes up about 40% to 60% of the TDD for most adults, so a 50/50 split is a reasonable starting point. The remaining half is divided among meals, usually in thirds.
For example, a 70 kg patient starting at 0.5 units/kg would have a TDD of 35 units. Half (roughly 17 to 18 units) goes to basal insulin. The other half is split across three meals, giving about 5 to 6 units of rapid-acting insulin before each meal. These numbers are starting estimates that require prompt fine-tuning based on glucose monitoring.
Calculating Mealtime Doses
Mealtime insulin dosing has two components: a carbohydrate coverage dose and a correction dose for elevated blood glucose.
Carbohydrate Coverage (Rule of 500)
The insulin-to-carbohydrate ratio tells the patient how many grams of carbohydrate one unit of rapid-acting insulin will cover. To calculate it, divide 500 by the TDD. A patient with a TDD of 50 units would get 500 ÷ 50 = 10, meaning 1 unit of insulin covers 10 grams of carbohydrate. As a general reference, one unit of rapid-acting insulin covers roughly 12 to 15 grams of carbohydrate in an average patient, but individual ratios can range from 4 to 30 grams.
This ratio often varies across the day. Insulin resistance tends to be higher in the morning due to hormonal patterns, so a patient might need a ratio of 1:8 at breakfast, 1:15 at lunch, and 1:12 at dinner. Establishing these patient-specific ratios takes time and careful tracking of post-meal glucose.
Correction Doses (1800 Rule)
The insulin sensitivity factor (ISF) tells you how much one unit of rapid-acting insulin will lower blood glucose. To calculate it in mg/dL, divide 1800 by the TDD. A patient with a TDD of 50 units would get 1800 ÷ 50 = 36, meaning one unit of rapid-acting insulin lowers blood glucose by about 36 mg/dL.
This correction factor is used when pre-meal glucose is above target. If the patient’s blood glucose is 210 mg/dL and their target is 120 mg/dL, the difference is 90 mg/dL. Dividing 90 by their ISF of 36 gives 2.5 units of correction insulin, added on top of the carbohydrate coverage dose.
Titrating Basal Insulin
The initial dose is rarely the right dose. The ADA recommends adjusting basal insulin by 10% to 15%, or 2 to 4 units, once or twice weekly until fasting blood glucose reaches target. As the target is approached, smaller and less frequent adjustments reduce hypoglycemia risk.
A more granular titration schedule based on three consecutive days of fasting glucose readings works like this:
- Above 180 mg/dL: increase by 8 units
- 160 to 180 mg/dL: increase by 6 units
- 140 to 159 mg/dL: increase by 4 units
- 120 to 139 mg/dL: increase by 2 units
- 100 to 119 mg/dL: increase by 1 unit
- 80 to 99 mg/dL: decrease by 1 unit
- 60 to 79 mg/dL: decrease by 2 units
- Below 60 mg/dL: decrease by 4 units
Adjustments should be made every three to four days at most, giving the new dose time to show its effect on fasting levels. Patient-assisted titration, where patients adjust their own basal dose according to a written algorithm, has been shown to be effective and improves time to target.
Adding Mealtime Insulin
If A1C or post-meal glucose remains above goal after basal insulin has been optimized (fasting glucose is at target but A1C is still elevated), the next step is adding rapid-acting insulin at meals. The most common approach is “basal-plus,” starting with one injection before the largest meal of the day and expanding to additional meals as needed. This stepwise approach minimizes injection burden and hypoglycemia while the patient adjusts.
When transitioning to full basal-bolus therapy, the basal dose may need to be reduced to avoid stacking. The total daily insulin requirement generally stays similar, but the distribution shifts as mealtime insulin takes on more of the work.
Managing Hypoglycemia Risk
Every insulin prescription should include a hypoglycemia action plan. Patients need to know that a blood glucose below 70 mg/dL requires immediate treatment using the 15-15 rule: consume 15 grams of fast-acting carbohydrate (glucose tablets, juice, or regular soda), wait 15 minutes, then recheck. If glucose remains below 70, repeat the process. Once blood glucose is back in range, follow up with a balanced snack containing protein and carbohydrate.
For patients at risk of severe hypoglycemia (inability to self-treat), prescribe emergency glucagon alongside insulin. Nasal glucagon delivers a 3 mg dry powder dose through a single-use device that requires no reconstitution and has shown efficacy comparable to injectable glucagon in clinical trials. Injectable formulations are also available, but the nasal form significantly reduces handling errors, which is critical when a caregiver or family member needs to administer it during an emergency.
Needle Selection and Delivery Devices
For patients using insulin pens, the recommended needle length is 4 to 6 mm for all adults, regardless of body mass index. International guidelines state there is no medical reason to recommend needles longer than 8 mm. Shorter needles (4 mm) cause significantly less injection pain compared to 8 mm needles and are equally effective at delivering insulin subcutaneously. Needles 4 to 6 mm should be inserted at 90 degrees. If longer needles are used, injecting at a 45-degree angle or using a skin fold helps avoid intramuscular injection, which causes erratic glucose absorption.
For patients who prefer not to inject, or who need many daily injections, insulin pumps deliver rapid-acting insulin continuously through a small catheter. The pump replaces both basal and bolus injections but requires more training and engagement from the patient.
What to Include on the Prescription
An insulin prescription needs to cover more than the insulin itself. The complete order includes the insulin formulation and concentration, pen or vial, needles or syringes (with gauge and length specified), a blood glucose meter, test strips, lancets, a lancing device, and a sharps disposal container. The quantity of test strips depends on the regimen: patients on basal-only insulin may test once or twice daily, while those on basal-bolus therapy typically need four or more tests per day.
Specify the number of injections per day and total daily units on the prescription so the pharmacy can calculate supply quantities accurately. For example, a patient using 40 units of basal insulin daily plus a pen needle per injection needs at least 30 pen needles per month for basal alone. Adding mealtime insulin and correction doses increases the needle count significantly. Including “allow for additional correction doses” or a specific daily maximum on the prescription prevents patients from running short.