Clinical guide comparing Vetsulin, ProZinc, glargine, and NPH for dogs and cats. Covers U-40 vs U-100 syringe safety math, openFDA data, storage, and switching rules.

When a dog or cat is diagnosed with diabetes mellitus, initiating insulin therapy is the cornerstone of clinical management. However, veterinarians and pet owners must navigate a complex choice of insulin formulations, species-specific pharmacokinetics, concentration differences, and strict hardware requirements.

Unlike human diabetes care where standard U-100 insulin is universal, veterinary practice utilizes both U-40 (40 IU/mL) veterinary-approved insulins and U-100 (100 IU/mL) human insulins used off-label. Using the wrong syringe for a given insulin concentration introduces a severe dosing error that can lead to fatal hypoglycemia.

This guide evaluates the primary insulin products used in veterinary medicine: Vetsulin (porcine insulin zinc suspension), ProZinc (protamine zinc recombinant human insulin), Glargine (Lantus), NPH (Humulin N / Novolin N), and Detemir (Levemir). It details pharmacokinetics by species, explains the U-40 vs. U-100 syringe matching math, analyzes FDA drug approval and openFDA adverse event data, outlines glucose monitoring options, and provides comprehensive storage and transition protocols.

Direct Answer: Quick Selection & Safety Summary

For veterinary clinicians and pet owners managing canine or feline diabetes:

Diabetic Dogs (First-Choice Insulin): Vetsulin (porcine lente insulin, U-40) is FDA-approved for dogs and matches canine insulin amino acid sequence identically. ProZinc (protamine zinc, U-40) is also FDA-approved for dogs and provides a longer duration of action if Vetsulin duration is inadequate.
Diabetic Cats (First-Choice Insulin): ProZinc (U-40) is FDA-approved for cats and offers predictable twice-daily glycemic control. Insulin Glargine (Lantus, U-100 off-label) is the primary long-acting human insulin choice for cats, demonstrating diabetic remission rates of 60% to 80%+ when started early alongside home blood glucose monitoring.
The Non-Negotiable Hardware Rule: Always match syringe unit scale to insulin concentration. U-40 insulin (Vetsulin, ProZinc) MUST be drawn only with U-40 syringes (red cap). U-100 insulin (Glargine, NPH, Detemir) MUST be drawn only with U-100 syringes (orange cap).
- Critical Math: Drawing a U-100 insulin into a U-40 syringe delivers 2.5 times the intended dose (a 250% overdose). Drawing a U-40 insulin into a U-100 syringe delivers only 40% of the intended dose (severe underdosing).

Concentration	Insulins in this category	Syringe to use
U-40 (40 IU/mL)	Vetsulin (porcine lente), ProZinc (protamine zinc)	Red-cap U-40 syringes only
U-100 (100 IU/mL)	Glargine/Lantus, NPH/Humulin N, Detemir/Levemir (all off-label human insulins)	Orange-cap U-100 syringes only

Never mix the hardware: drawing a U-100 insulin into a U-40 syringe delivers a 2.5× overdose.

Comparison Matrix: Veterinary & Human Insulins for Pets

Insulin Product	Active Ingredient	Concentration	FDA Animal Approval	Dog Duration (Nadir)	Cat Duration (Nadir)	Key Clinical Fit
Vetsulin (Intervet / Merck)	Porcine insulin zinc suspension	U-40 (40 IU/mL)	FDA Approved (NADA 141-236, Dogs & Cats)	8 – 14 hrs (Nadir 4–8h)	6 – 10 hrs (Nadir 2–6h)	First-choice in dogs; intermediate-acting lente formulation.
ProZinc (Boehringer Ingelheim)	Protamine zinc recombinant human insulin	U-40 (40 IU/mL)	FDA Approved (NADA 141-297, Cats 2009, Dogs 2019)	10 – 18 hrs (Nadir 5–12h)	10 – 14 hrs (Nadir 4–8h)	First-choice in cats; long-acting PZI for twice-daily dosing.
Glargine (Lantus / Sanofi)	Recombinant human insulin analogue	U-100 (100 IU/mL)	Off-Label (Human approved)	8 – 12 hrs (Nadir 4–8h)	12 – 24 hrs (Nadir 8–14h)	Remission choice in cats; long-acting peakless baseline micro-precipitate.
NPH (Humulin N / Novolin N)	Isophane recombinant human insulin	U-100 (100 IU/mL)	Off-Label (Human approved)	6 – 10 hrs (Nadir 2–6h)	< 6 hrs (Rapid clearance)	Budget dog option; Not recommended in cats (duration too short).
Detemir (Levemir / Novo Nordisk)	Recombinant human insulin analogue (fatty acid chain)	U-100 (100 IU/mL)	Off-Label (Human approved)	12 – 24 hrs (High potency)	12 – 24 hrs (Nadir 8–12h)	Ultra-potent in dogs (start at 0.1 IU/kg); long-acting cat alternative.

Veterinary Product Profiles & FDA Label Analysis

Official FDA CVM approval records and DailyMed animal drug labels establish the exact regulatory status and formulation specifications for veterinary insulins:

1. Vetsulin (Porcine Insulin Zinc Suspension)

FDA Regulatory Status: Approved under NADA 141-236 (Sponsor: Intervet Inc. / Merck Animal Health). First approved for dogs; expanded to cats. Greenbook records republished May 14, 2026; DailyMed SPL label effective June 30, 2025.
Formulation & Structure: Porcine-derived insulin consisting of 35% amorphous and 65% crystalline zinc insulin in aqueous suspension. Porcine insulin shares an identical amino acid sequence with canine insulin, minimizing anti-insulin antibody formation in dogs. (Feline insulin differs by 3 amino acids from porcine/canine).
Concentration & Hardware: U-40 (40 IU/mL). Must be administered with dedicated U-40 syringes or the Vetsulin VetPen.
Handling & In-Use Stability: Must be shaken thoroughly until a uniform milky suspension is obtained before drawing. Store refrigerated (2^°C to 8^°C). The official label limits in-use vial stability to 42 days after first puncture.

2. ProZinc (Protamine Zinc Recombinant Human Insulin)

FDA Regulatory Status: Approved under NADA 141-297 (Sponsor: Boehringer Ingelheim Animal Health). Approved for cats in 2009; supplemental approval for dogs granted by FDA CVM in 2019. Greenbook record updated November 13, 2023; DailyMed label effective January 22, 2025.
Formulation & Structure: Recombinant human insulin conjugated with protamine and zinc to delay absorption, creating a long-acting depot effect.
Concentration & Hardware: U-40 (40 IU/mL). Uses U-40 red-cap syringes.
Handling & In-Use Stability: Must be gently rolled between palms (do not shake vigorously) to resuspend prior to use. Refrigerate (2^°C to 8^°C). In-use vial may be maintained up to 30^°C (86^°F) during daily use, with discard at 60 days.

Human Insulins Used Off-Label in Small Animal Practice

When FDA-approved veterinary insulins are insufficient or when feline remission is the primary goal, human insulins are prescribed under AMDUCA (Animal Medicinal Drug Use Clarification Act) off-label guidelines.

1. Insulin Glargine (Lantus, Toujeo, Semglee)

Glargine is a soluble human insulin analogue produced by recombinant DNA technology. When injected subcutaneously into the acidic solution (pH 4.0), it forms micro-precipitates in the neutral subcutaneous tissue, releasing small amounts of insulin slowly over time.

Feline Diabetic Remission Evidence: In feline diabetes, glargine administered twice daily alongside a low-carbohydrate wet diet yields exceptional remission rates. Clinical trials (Bloom et al.) demonstrate overall diabetic remission rates of 64%, rising to 84% when glargine is initiated within 6 months of initial diagnosis.
Concentration & Syringe: U-100 (100 IU/mL). Requires orange-cap U-100 0.3 mL half-unit marked insulin syringes.
Handling: Clear solution; do NOT shake or roll. Store refrigerated. Do not dilute.

2. Insulin Detemir (Levemir)

Detemir is a long-acting human insulin analogue with a fatty acid chain (myristic acid) bound to position B29. This allows it to bind reversibly to albumin in the subcutaneous tissue and bloodstream, creating a prolonged release kinetics.

High Potency Caution in Dogs: Detemir is supplied at about four times the molar concentration of other insulins (about 24 nmol per unit versus 6 nmol per unit for NPH or glargine) and is roughly equipotent to human insulin per molecule in dogs. The net effect is that detemir is about 4 times more potent per unit in dogs than Vetsulin or NPH, so the starting dose must be reduced to 0.1 IU/kg SC q12h (versus 0.25–0.5 IU/kg for Vetsulin), with particular caution in small dogs, which are more prone to hypoglycemia.
Concentration: U-100 (100 IU/mL).

3. NPH (Humulin N, Novolin N)

NPH (Neutral Protamine Hagedorn) is an intermediate-acting human insulin suspension.

Clinical Fit: Historically used as a low-cost option for diabetic dogs. In dogs, NPH has a shorter duration of action than Vetsulin (often requiring strict 12-hour dosing with rapid glucose drops). In cats, NPH is rapidly metabolized (duration <6 hours) and is clinically ineffective for long-term feline management.
Concentration: U-100 (100 IU/mL).

The U-40 vs U-100 Syringe Safety Math: Preventing Fatal Dosing Errors

The co-existence of U-40 and U-100 insulins in veterinary medicine is one of the most hazardous drug administration traps in clinical practice.

Because insulin concentration is stated in units per milliliter, 1.0 mL of a U-40 insulin holds 40 IU of active drug, while 1.0 mL of a U-100 insulin holds 100 IU — U-100 insulin is 2.5 times more concentrated than U-40. The unit marks on a syringe are calibrated to exactly one of those concentrations, never both.

Demonstrating the Syringe Mismatch Error

Case 1: U-100 Insulin drawn into a U-40 Syringe (The Overdose Trap)

If a pet owner or technician takes a vial of U-100 Glargine and draws it to the "4 Unit" line on a U-40 syringe:

A U-40 syringe mark for 4 units represents a volume of 4 ÷ 40 = 0.10 mL.
Drawing 0.10 mL of U-100 insulin yields: 0.10 mL × 100 IU/mL = 10 International Units.
Result: The patient receives 10 Units instead of 4 Units — a 2.5x (250%) severe overdose, resulting in acute, life-threatening hypoglycemic shock, seizures, and potential brain damage. Peer-reviewed case reports (PMC6601479) document near-fatal ICU admissions from this exact mismatch.

Case 2: U-40 Insulin drawn into a U-100 Syringe (The Underdose Trap)

If an owner takes a vial of U-40 Vetsulin and draws it to the "10 Unit" mark on a U-100 syringe:

A U-100 syringe mark for 10 units represents a volume of 10 ÷ 100 = 0.10 mL.
Drawing 0.10 mL of U-40 insulin yields: 0.10 mL × 40 IU/mL = 4 International Units.
Result: The patient receives only 40% of the prescribed dose, leading to persistent hyperglycemia, polyuria/polydipsia, and progression toward diabetic ketoacidosis. For details on DKA management, see our emergency guide on diabetic ketoacidosis in dogs and cats.

openFDA Adverse Event Scale & Drug Safety Data

Insulin is a high-alert biologic. An inspection of the official openFDA Animal Drug Adverse Event dataset (export through July 3, 2026, comprising 1,357,337 total animal drug safety reports) confirms that injectable insulin products rank among the most frequently reported veterinary drug classes.

FDA Animal Drug Adverse Event Report Volume

Ingredient Listing (openFDA Active Ingredient Dimension)	Total Adverse Event Report Mentions	Dominant Reported Adverse Reactions
Insulin Injectable Vial (General Category)	5,153	Hypoglycemia, ataxia, lethargy, seizures, lack of efficacy
Porcine Insulin (Vetsulin / Caninsulin)	2,117	Hypoglycemic shock, rebound hyperglycemia (Somogyi), injection site pain
Protamine Zinc Recombinant Human Insulin (ProZinc)	1,869	Hypoglycemia, variable duration, lethargy, vomiting
Insulin Glargine (Lantus / Off-label)	67	Accidental overdose (syringe mismatch), localized reaction

Note on OpenFDA Data Interpretation: In accordance with FDA CVM guidelines and our overview of how FDA animal adverse-event reports are counted, adverse event report counts reflect passive surveillance data. Brand names are masked in active-ingredient tables, a single report can mention multiple concurrent medications, and report volume reflects widespread chronic use rather than inherent drug toxicity. Hypoglycemia remains the primary adverse event across all insulin formulations.

For a dedicated analysis of feline-specific hypoglycemia risk factors and glucose curve monitoring, consult our guide on feline insulin adverse-event and hypoglycemia analysis as well as the feline diabetes monitoring guide.

Glucose Monitoring SOP: CGMs vs Point-of-Care Blood Glucose Curves

Effective insulin regulation depends on reliable glucose monitoring. The 2026 AAHA Diabetes Management Guidelines strongly emphasize home monitoring over in-clinic glucose curves to eliminate stress-induced hyperglycemia (particularly in cats).

1. Continuous Glucose Monitors (CGMs: FreeStyle Libre 2/3)

Continuous glucose monitors have revolutionized veterinary endocrinology by recording interstitial glucose readings every 1 to 5 minutes for up to 14 days.

Sensor Placement: Applied to the dorsal neck or interscapular region. Hair must be shaved cleanly, skin prepped with alcohol, and secured with liquid adhesive (e.g., Skin-Tac) or a protective body wrap.
Clinical Benefits: Provides complete 24-hour glucose trend curves, identifying the exact glucose nadir (peak insulin effect) and duration of action without hospital stress.
Lag Time & Calibration: Interstitial fluid glucose lags blood glucose by 10 to 15 minutes during rapid glucose swings. In severe hypoglycemia (< 60 mg/dL), verify CGM readings with a marginal ear-vein blood glucose meter before treating.

2. Point-of-Care Blood Glucose Meters (AlphaTRAK 3)

Veterinary-Calibrated Meters: Always use a veterinary-calibrated meter (such as AlphaTRAK 3). Human handheld meters (e.g., OneTouch, Contour) underestimate animal blood glucose because glucose is distributed differently between plasma and red blood cells in dogs/cats versus humans (human RBCs contain ~40% of glucose, whereas feline RBCs contain only ~15%).
Sampling Sites: Marginal ear vein (pinna), paw pad, or lip mucosa.

Troubleshooting Uncontrolled Diabetes: Somogyi Effect vs Insulin Resistance

When a diabetic patient remains polyuric and glucosuric despite increasing insulin doses, clinicians must differentiate between the Somogyi Phenomenon (rebound hyperglycemia) and True Insulin Resistance:

Run a 12-hour glucose curve (or use a CGM) and read the nadir:

Curve pattern	Diagnosis	Why it happens	Action
Nadir < 65 mg/dL, then a rapid spike above 300 mg/dL	Somogyi (rebound)	Insulin-induced hypoglycemia triggers counter-regulatory hormones (epinephrine, glucagon, cortisol)	Reduce the insulin dose by 25–50%
Nadir stays above 150 mg/dL across the whole 12 hours	True insulin resistance	Underlying driver: acromegaly, hyperadrenocorticism, infection (dental/urinary), pancreatitis, or steroid/progestin use	Work up the cause and increase the dose

The Somogyi Phenomenon: Occurs when an excessive insulin dose causes blood glucose to drop rapidly below 65 mg/dL (3.6 mmol/L). The brain senses acute neuroglycopenia and triggers a massive release of counter-regulatory hormones (epinephrine, glucagon, cortisol, growth hormone). These hormones induce rapid hepatic glycogenolysis and gluconeogenesis, driving blood glucose up to 300 - 500 mg/dL within hours.
- The Clinical Trap: An owner checking a single spot glucose test in the evening sees 450 mg/dL and incorrectly assumes the pet needs more insulin. Increasing the dose worsens hypoglycemia and exacerbates rebound spikes.
- Correct Action: Perform a full curve to catch the early nadir. Decrease the insulin dose by 25% to 50%.
True Insulin Resistance: Defined as requiring > 1.5 IU/kg per dose in dogs or > 2.0 IU/kg per dose in cats to maintain glycemic control. Common underlying causes include acromegaly (feline hypersomatotropism, present in ~15-25% of diabetic cats), hyperadrenocorticism (Cushing's in dogs), systemic infections (pyelonephritis, severe periodontitis), pancreatitis, or concurrent administration of glucocorticoids/progestins.

Canine vs Feline Insulin Protocols: Choice by Species

Patient	First choice	Alternative
Canine (Type 1-like; remission rare)	Vetsulin (U-40), 0.25–0.5 IU/kg q12h	ProZinc (U-40) if Vetsulin duration is too short
Feline (Type 2-like; remission possible)	Assess SGLT2 suitability first (Senvelgo/Bexacat); if insulin is needed, ProZinc (U-40) or glargine (U-100), 0.25–0.5 IU/kg q12h	Glargine maximizes remission odds with tight home monitoring

Dog-Specific Rules

Pathophysiology: Canine diabetes is almost exclusively insulin-dependent (similar to human Type 1 diabetes), resulting from immune-mediated destruction of pancreatic beta cells or secondary to pancreatitis. Diabetic remission in dogs is extremely rare. Lifetime insulin therapy is required.
First Choice: Vetsulin is the traditional first choice. Initial starting dose is typically 0.25 to 0.5 IU/kg SC q12h (given immediately after a meal).
Alternative: If Vetsulin duration is under 8 hours (causing end-of-dose hyperglycemia before the next injection), switch to ProZinc at 0.5 IU/kg q12h.

Cat-Specific Rules

Pathophysiology: Feline diabetes resembles human Type 2 diabetes, characterized by peripheral insulin resistance and beta-cell amyloid deposition. Early aggressive glycemic control can induce diabetic remission.
First Choice: ProZinc (U-40) or Glargine (U-100) at 0.25 to 0.5 IU/kg SC q12h.
Oral SGLT2 Inhibitor Alternative: In newly diagnosed diabetic cats that are eating well and are NOT ketotic or previously treated with insulin, oral SGLT2 inhibitors like Bexacat or Senvelgo provide an oral non-insulin alternative. Read our detailed reviews on Senvelgo and the SGLT2 option for cats and the SGLT2 inhibitor adverse-event data for cats.

Dietary Integration & Insulin Timing SOP

Insulin administration must be integrated with daily feeding schedules to prevent hypoglycemia and postprandial glucose surges:

Canine Dietary Management

Diet Composition: High-fiber, complex-carbohydrate, low-fat prescription diets (e.g., Hill's w/d, Royal Canin Glycobalance). Fiber slows intestinal glucose absorption and minimizes postprandial blood glucose spikes.
Feeding Schedule: Feed two equal meals per day, spaced exactly 12 hours apart. Administer insulin immediately after or during the meal. Never inject insulin before verifying that the dog has consumed its meal. If the dog eats only half its meal, administer half the insulin dose; if the dog refuses to eat, withhold insulin and contact the clinic.

Feline Dietary Management

Diet Composition: High-protein, ultra-low-carbohydrate canned/wet diets (< 10% metabolizable energy from carbohydrates, e.g., Purina Pro Plan DM, Hill's m/d, Fancy Feast Classic Pate). Low-carbohydrate diets dramatically decrease insulin demand and promote diabetic remission.
Feeding Schedule: Cats are natural nibblers. Because long-acting insulins (ProZinc, Glargine) provide smooth baseline coverage, cats may be fed multiple small meals throughout the day or allowed to graze, provided total daily caloric intake is controlled.

Storage, Handling, and Transition SOPs

Storage & Handling Summary Rules

Temperature Control: Unopened vials of Vetsulin, ProZinc, Glargine, NPH, and Detemir MUST be stored refrigerated between 2^°C and 8^°C (36^°F to 46^°F). Never freeze insulin. Freezing denatures the protein structure and destroys efficacy.
Agitation Rules:
- Vetsulin: Shake vigorously until a smooth, uniform, milky suspension forms.
- ProZinc & NPH: Gently roll the vial between palms. Do not shake.
- Glargine & Detemir: Clear solutions. Do not shake or roll.
In-Use Discard Windows:
- Vetsulin: Discard 42 days after first puncture per FDA label.
- ProZinc: Discard 60 days after first puncture.
- Glargine (Lantus): Human label states 28 days at room temp; veterinary consensus permits refrigerated use for 60 to 90 days as long as solution remains clear without precipitation.

How Veterinarians Safely Switch Insulins

When switching a pet from one insulin product to another (e.g., from Vetsulin to ProZinc, or from NPH to Vetsulin due to duration failure):

Reduce the Starting Dose: Never transfer unit-for-unit when changing insulin formulations. Reduce the total daily dose by 25% to 50% when initiating the new product to prevent accidental rebound hypoglycemia.
Equip New Syringes: Verify that the owner leaves the clinic with the correct syringe type (red cap U-40 vs. orange cap U-100) corresponding to the new insulin. Retrieve and dispose of old syringes.
Stabilization Period: Allow 5 to 7 days on the new insulin formulation before performing a full 12-hour blood glucose curve or adjusting the dose, as tissue depot kinetics require several days to reach steady-state.

Frequently Asked Questions

Can I use human insulin like Lantus or Humulin N for my pet?

Yes, but under veterinary direction. Glargine (Lantus) is widely used off-label in diabetic cats with excellent remission results. NPH (Humulin N) is sometimes used in dogs as a budget option. However, human insulins are U-100 concentration, meaning you must use U-100 syringes, and their duration of action differs from veterinary-approved U-40 insulins like Vetsulin and ProZinc.

What happens if I use a U-100 syringe with U-40 Vetsulin?

If you draw U-40 Vetsulin into a U-100 syringe, you will severely underdose your pet. You will deliver only 40% of the intended insulin dose, causing blood glucose levels to remain dangerously high. Conversely, using a U-40 syringe with U-100 insulin delivers a 2.5x overdose, which can cause fatal hypoglycemic shock.

Can a diabetic cat achieve remission, and which insulin is best?

Yes. Up to 60% to 80% of diabetic cats can achieve diabetic remission (meaning they maintain normal blood glucose levels without insulin injections for months or years) if diagnosed early and treated with a long-acting insulin—such as Glargine or ProZinc—combined with a strict low-carbohydrate wet food diet.

How much does monthly insulin therapy cost for a dog or cat?

A 10 mL vial of Vetsulin or ProZinc costs between $60 and $120 and typically lasts 2 to 3 months for a cat or small dog, making drug costs around $25 to $40 per month. For a large 40 kg dog, insulin consumption is higher, costing $70 to $120 per month. Syringes cost approximately $15 to $25 per box of 100.

Why is detemir (Levemir) so much more potent in dogs than in humans?

Detemir is supplied at about four times the molar concentration of other insulins (about 24 nmol per unit versus 6 nmol per unit for NPH or glargine). Because detemir is roughly equipotent to human insulin per molecule in dogs, each unit delivers far more active insulin than a unit of Vetsulin or NPH. The net effect is that detemir is about 4 times more potent per unit in dogs than in humans, requiring a starting dose of just 0.1 IU/kg SC q12h — with extra caution in small dogs.

Sources

FDA Greenbook & DailyMed Labels: U.S. Food and Drug Administration Center for Veterinary Medicine. Approved Animal Drug Products (Greenbook) for NADA 141-236 (Vetsulin) and NADA 141-297 (ProZinc). DailyMed National Library of Medicine SPL Labels. Animal Drugs @ FDA
AAHA Diabetes Guidelines: Behrend E, Holford A, Lathan P, et al. 2018 AAHA Diabetes Management Guidelines for Dogs and Cats. Journal of the American Animal Hospital Association. 2018;54(1):1-21. AAHA Guidelines
AAHA Feline Diabetes Guidelines: 2026 AAHA Diabetes Management Guidelines for Cats. Journal of the American Animal Hospital Association. 2026. AAHA Feline Resource
Syringe Mismatch Clinical Case Report: Service C, et al. Dosing errors associated with improper syringe selection for U-40 and U-100 veterinary insulins. PubMed Central. PMC6601479
openFDA Animal Drug Adverse Event Dataset: U.S. Food and Drug Administration. openFDA Animal Adverse Event API & dim_active_ingredient dataset. Data export current through July 3, 2026. openFDA Animal Events
Feline Glargine Remission Study: Bloom CA, et al. Feline diabetes mellitus and insulin glargine: Remission rates and clinical outcomes. Journal of Feline Medicine and Surgery. 2024. PMC11383081
Merck Animal Health Vetsulin Technical Manual: Merck & Co., Inc. Vetsulin (porcine insulin zinc suspension) Product Monograph, Dosing & Transition Guidance. Vetsulin Official Site

Vet Med Guide

Insulin for Diabetic Dogs and Cats: Vetsulin vs ProZinc vs Glargine vs NPH