Approximately how many 1.8 mL cartridges of lidocaine 2% with 1:100,000 epinephrine are equivalent to the typical maximum recommended dose for a 70 kg adult?

• A. Ten cartridges
• B. About 14 cartridges
• C. Twenty cartridges
• D. Twenty-five cartridges

Answer: (B)

The main idea is to translate the weight-based maximum dose of lidocaine with epinephrine into how many 1.8 mL cartridges it would equal. For lidocaine with epinephrine, a common safety limit is about 7 mg per kilogram, up to roughly 500 mg for an adult. For a 70 kg person, that’s 70 × 7 = 490 mg (rounded to about 500 mg). Each 1.8 mL cartridge of 2% lidocaine with epinephrine contains 20 mg/mL × 1.8 mL = 36 mg of lidocaine. Dividing the maximum safe total (about 490–500 mg) by per-cartridge dose (36 mg) gives 490 ÷ 36 ≈ 13.6, so the practical amount is about 14 cartridges. The other options would exceed the safe limit (20 cartridges = 720 mg; 25 cartridges = 900 mg) or stay well under it (10 cartridges = 360 mg). Epinephrine helps by reducing systemic uptake, but the weight-based dose limit still guides how many cartridges are considered safe.