Short answer: To calculate a dental local anaesthetic maximum dose, multiply the patient’s weight by the mg per kg limit, apply any cap, then divide by the milligrams in one cartridge and round down. A 2 percent solution holds 20 mg in each mL, so a 1.8 mL cartridge holds 36 mg.
Why dental exams like this calculation
DHA’s guideline for a General Dentist lists twelve content areas, and the first is anaesthesia and pain management. DHA publishes no weights and no sample questions, so we cannot say how often a calculation appears. It is a reasonable guess, labelled as our inference, that a topic with a number in it is easy to turn into a question.
Meddex is independent and is not connected to the Dubai Health Authority (DHA) or Prometric.
This is exam-preparation revision, not clinical advice. In practice, follow your own organisation’s policies, the product label and the current guideline, and check every dose and threshold against them.
Step 1: turn the percentage into milligrams
A solution labelled 1 percent holds 1 g in 100 mL, which is 10 mg in each mL. So multiply the percentage by 10 to get mg per mL. A 2 percent solution is 20 mg per mL, and a 4 percent solution is 40 mg per mL.
A dental cartridge usually holds 1.8 mL, although some hold 2.2 mL. Read the volume in the question. Then milligrams per cartridge is mg per mL multiplied by the volume: 20 times 1.8 is 36 mg for lidocaine 2 percent, and 40 times 1.8 is 72 mg for articaine 4 percent.
Step 2: use the limit the question gives you
Textbooks quote maximum doses in mg per kg, and sources do not agree exactly. Commonly quoted figures for adults are about 4.4 mg per kg for lidocaine without a vasoconstrictor, about 7 mg per kg for lidocaine with adrenaline and about 7 mg per kg for articaine, often with an absolute ceiling per patient. Because the figures differ between texts and labels, a fair question states the limit it wants you to use. Use the number in the stem, never the one you remember.
Weigh the patient in kilograms, not pounds. Apply the ceiling if the answer is higher. Children and light adults reach the limit with fewer cartridges than people expect, which is the real lesson of this topic.
Example 1: an adult with a ceiling
Example, written by Meddex with invented limits. An adult weighs 70 kg. The question says use 4.4 mg per kg with a ceiling of 300 mg for lidocaine 2 percent. 70 times 4.4 is 308 mg, which is above the ceiling, so the limit is 300 mg. Each 1.8 mL cartridge holds 36 mg. 300 divided by 36 is 8.33. Round down: 8 cartridges at most, which is 288 mg.
Always round down. Nine cartridges would be 324 mg, which is over the limit.
Example 2: a child
Example, written by Meddex. A child weighs 20 kg and the stem says use 4.4 mg per kg for lidocaine 2 percent with adrenaline. 20 times 4.4 is 88 mg. 88 divided by 36 is 2.44, so 2 cartridges, or 72 mg. Two cartridges is a small amount of anaesthetic, which is why paediatric dental work calls for careful planning and for a technique that uses the least volume that works.
Example 3: articaine
Example, written by Meddex. An adult weighs 60 kg and the stem gives 7 mg per kg for articaine 4 percent. 60 times 7 is 420 mg. A 1.8 mL cartridge holds 72 mg. 420 divided by 72 is 5.83, so 5 cartridges, or 360 mg. Check the unit conversion once more before you answer: 4 percent is 40 mg per mL, and 40 times 1.8 is 72.
Adrenaline: the second limit
A ratio of 1 in 100,000 means 1 g in 100,000 mL, which is 0.01 mg or 10 micrograms in each mL. A 1.8 mL cartridge of 1 in 100,000 therefore holds 18 micrograms. A 1 in 200,000 cartridge holds 9 micrograms.
Teaching for patients with significant cardiovascular disease is to keep adrenaline to about 40 micrograms at one visit, with the prescriber’s advice where the disease is unstable. Example, written by Meddex: 40 divided by 18 is 2.2, so no more than 2 cartridges of 1 in 100,000. Questions on this topic often ask you to choose between a plain solution, a lower adrenaline concentration and deferring treatment, and the safest answer is usually the one that limits adrenaline and treats the cause of pain without delay.
Recognising toxicity
Early signs of local anaesthetic toxicity are numbness around the mouth, a metallic taste, light-headedness, ringing in the ears, slurred speech and muscle twitching. Later signs are seizures, loss of consciousness and cardiovascular collapse. Prevention is simple: aspirate before injecting, inject slowly, stay within the limit and ask about the patient’s weight and medicines.
If you suspect toxicity, stop injecting, call for help, give oxygen, protect the airway and treat seizures. Severe cases are managed with local protocols that include intravenous lipid emulsion. Read the DHA exam for dentists page for the official content areas, and the dentist revision plan for how to fit this around the rest of the list.
How to practise
Make five cards, each with a different drug, weight and cartridge size, and solve them on paper with the units written beside every number. Check your habit with the dosage calculations guide, which shows the same routine for other medicines. For the wider picture of the title’s format and pass score, see DHA exam format and pass mark by title.
Frequently asked questions
This article is general educational guidance, not official advice. Exam rules, fees and timelines are set by the Dubai Health Authority and the test provider and can change, so always confirm the latest details on their official channels. Review of Meddex content by named, licensed clinicians is not complete yet. See how we write and check our content.
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