Drug calculations can be scary for a veterinary technician, but it doesn’t have to be. Whether you’re building a CRI, calculating max nerve block doses, or reconstituting antibiotics, this guide is designed to help you feel confident and competent in the treatment area. We’ll walk through real-world examples of how to:
- Calculate drug doses
- Set up CRIs using syringe pumps or fluid bags
- Remove fluids when adding drugs
- Mix and dose FLK bags (Fentanyl-Lidocaine-Ketamine)
- Reconstitute injectable antibiotics like ampicillin and cefazolin
- Calculate regional nerve block doses and max safe volumes
Let’s break down these veterinary technician drug calculations one step at a time.
How to Calculate a Single Drug Dose
The formula is simple:
Dose (mg) = Weight (kg) × Dose (mg/kg)
Volume (mL) = Dose (mg) ÷ Concentration (mg/mL)
Example:
Give Hydromorphone 0.05 mg/kg IM to a 60 lb (27.3 kg) dog.
- 27.3 kg × 0.05 mg/kg = 1.37 mg
- Concentration: 2 mg/mL
- 1.37 mg ÷ 2 mg/mL = 0.685 mL
You’ll draw up 0.68 mL of hydromorphone.
How to Calculate CRIs
There are two main ways to give a constant rate infusion (CRI):
- Syringe pump CRI
- CRI in fluids (e.g., LRS + drug added to the bag)
Syringe Pump CRI
Use this formula:
mL/hr = (Weight × Dose × 60) ÷ Concentration
Let’s do a fentanyl CRI at 5 mcg/kg/hr for a 25 kg dog with a 50 mcg/mL syringe:
- 5 mcg/kg/hr ÷ 60 = 0.083 mcg/kg/min
- 0.083 × 25 × 60 = 124.5 mcg/hr
- 124.5 ÷ 50 = 2.49 mL/hr
Set the pump to 2.5 mL/hr
CRI in a Fluid Bag
Let’s make a lidocaine CRI for a 20 kg dog, 50 mcg/kg/min, using a 500 mL bag of fluids.
- Dose in mcg:
50 × 20 × 60 = 60,000 mcg = 60 mg - Lidocaine concentration = 20 mg/mL
60 ÷ 20 = 3 mL of lidocaine
Remove 3 mL from the fluid bag before adding
Run the bag at 500 mL/hr to deliver over 1 hour
FLK Bag Recipe (Fentanyl-Lidocaine-Ketamine)
Let’s build a 1000 mL LRS bag containing:
- Fentanyl: 20 mcg/mL
- Lidocaine: 1.2 mg/mL
- Ketamine: 1 mg/mL
Step-by-Step:
- Decide how much drug you want per mL
- Fentanyl: 20 mcg/mL × 1000 = 20,000 mcg = 20 mg
- Lidocaine: 1.2 mg/mL × 1000 = 1200 mg
- Ketamine: 1 mg/mL × 1000 = 1000 mg
- Convert to volume:
- Fentanyl (50 mcg/mL): 20,000 ÷ 50 = 400 mL
- Lidocaine (20 mg/mL): 1200 ÷ 20 = 60 mL
- Ketamine (100 mg/mL): 1000 ÷ 100 = 10 mL
- Total volume added: 400 + 60 + 10 = 470 mL
- Remove 470 mL of LRS before adding drugs
Now you have 1000 mL of FLK bag at your target concentrations.
How to Dose:
If your patient needs:
- Fentanyl 5 mcg/kg/hr
- Lidocaine 50 mcg/kg/min
- Ketamine 10 mcg/kg/min
You can calculate the fluid rate needed based on these concentrations.
Let’s say your patient is 25 kg.
- Fentanyl dose: 5 × 25 = 125 mcg/hr
- Bag has 20 mcg/mL → 125 ÷ 20 = 6.25 mL/hr
So set the bag to 6.25 mL/hr to deliver all three drugs at once.
Reconstituting Injectable Antibiotics
Cefazolin
Most vials are 1 gram (1000 mg). Add 10 mL of sterile water to make a concentration of:
1000 mg ÷ 10 mL = 100 mg/mL
Draw your dose from this.
Example: 22 kg dog at 22 mg/kg → 22 × 22 = 484 mg
484 ÷ 100 = 4.84 mL
Ampicillin Sodium
Usually, it comes as a 1-gram vial. Add 5 mL of sterile water for a concentration of:
1000 mg ÷ 5 mL = 200 mg/mL
Example: 10 kg dog at 22 mg/kg → 220 mg dose
220 ÷ 200 = 1.1 mL
Always use sterile water (not saline)
Discard unused portions after 1–2 hours unless refrigerated
Regional Nerve Blocks and Max Doses
When doing infraorbital, mental, mandibular, or maxillary nerve blocks, it’s essential to stay under safe limits for lidocaine and bupivacaine.
Max Dose Guidelines:
| Drug | Max Dose (mg/kg) | Duration |
| Lidocaine | 4 mg/kg | 1–2 hours |
| Bupivacaine | 2 mg/kg | 4–8 hours |
Total dose = weight × max dose
Example:
The dog is 25 kg
Max bupivacaine dose: 25 × 2 = 50 mg
Bupivacaine = 0.5% (5 mg/mL)
50 ÷ 5 = 10 mL total max volume
Split this across multiple blocks. If doing all four quadrants, use ≤ 2.5 mL per site
(This would be for a max dose. You would never actually use 2.5 ml per site in a regional nerve block.)
Always aspirate first!
Avoid using the entire max dose unless all quadrants require blocking.
Final Tips
- Always double-check your units (mg vs mcg)
- Always calculate the total volume before adding to fluid bags
- When in doubt, ask someone to double-check your math
- Know your maximum doses before drawing up nerve blocks
Continuing Education for Veterinary Technicians and Assistants
A veterinary technician mastering drug calculations, CRIs, and antibiotic reconstitution is essential when they aim to provide top-tier patient care. These skills not only enhance treatment efficacy but also ensure safety and precision in every procedure. At Learn Veterinary Dentistry, we recognize the pivotal role technicians play in veterinary medicine and are dedicated to equipping you with the knowledge and hands-on experience needed to excel.
Contact us today to explore our comprehensive continuing education courses, designed specifically for veterinary technicians and assistants. Whether you’re looking to deepen your understanding of anesthesia protocols, refine your radiographic techniques, or advance your dental charting skills, our expert-led programs offer both lecture and lab components to reinforce learning. Join us in elevating the standard of care in veterinary dentistry.
Images used under creative commons license – commercial use (05/12/2025) Photo by Jonatan Bustos on Unsplash