3.3 Medication Administration & Dosage Math

The Rights of Medication Administration

Safe medication administration starts with the rights — commonly taught as the eight rights: right client, drug, dose, route, time, reason, documentation, and client response / right to refuse. Confirm the right client with two unique identifiers (for example, full name and date of birth) checked against the MAR (medication administration record) and the identification band — a room or bed number is never an identifier. Perform the three checks: read the label when you remove the drug from storage, when you prepare it, and again at the bedside before giving it. Verify allergies every time, and document after administration, never before.

Safe Administration Practices

Give only medications you personally prepared; never chart or administer a drug someone else drew up. Know each drug's purpose, usual dose range, and key adverse effects before giving it. Assess relevant parameters first: count the apical pulse for one full minute before digoxin and withhold the dose if it is below 60 bpm in an adult; check the respiratory rate before opioids and hold if it is below 12 (certainly below 8), keeping naloxone available; verify blood pressure before antihypertensives; and confirm the INR for warfarin (a common therapeutic target is 2.0 to 3.0). If an order is unclear, incomplete, or appears unsafe, clarify with the prescriber before acting rather than guessing.

Dosage Calculation — Worked Examples

The core oral and parenteral formula is Desired over Have times Quantity: Amount = (Desired dose / dose on Hand) x quantity. Always convert to the same unit first — 1 g = 1000 mg, 1 mg = 1000 mcg, 1 L = 1000 mL — because unit mismatch is a leading cause of medication error.

1. Oral tablets. Order 500 mg; stock 250 mg tablets. 500 / 250 = 2 tablets.

2. Parenteral volume with conversion. Order 1 g; vial labelled 500 mg/mL. Convert 1 g to 1000 mg, then 1000 / 500 = 2 mL.

3. IV drip rate (gtt/min). Formula: gtt/min = (volume in mL x drop factor in gtt/mL) / total time in minutes. Infuse 1000 mL over 8 hours with 15 gtt/mL tubing: (1000 x 15) / (8 x 60) = 15000 / 480 = 31.25, which rounds to 31 gtt/min. Always read the drop factor off the actual tubing, because it varies (10, 15, 20, or 60 gtt/mL microdrip).

4. IV pump rate (mL/h). 1000 mL over 8 hours: 1000 / 8 = 125 mL/h.

5. Weight-based dose. Order 5 mg/kg for a 70 kg adult: 5 x 70 = 350 mg per dose.

6. Pediatric safety check. A child weighs 20 kg; the safe range is 10 to 15 mg/kg/day divided every 6 hours. Safe daily range = 200 to 300 mg/day, so per dose (divided by 4) = 50 to 75 mg. If the order exceeds this, hold and clarify. Pediatric doses are frequently verified against mg/kg/day limits or body-surface area (BSA).

High-Alert Medications

High-alert drugs carry a heightened risk of serious harm if given in error and require an independent double-check by a second nurse. A useful memory group is insulin, heparin and other anticoagulants, opioids or narcotics, and concentrated electrolytes. Rules the exam tests: IV potassium chloride must be diluted and infused by a controlled pump — never given IV push, because rapid potassium causes fatal cardiac dysrhythmias; insulin is measured only in an insulin syringe, with type and timing clarified (rapid-acting aspart is given within about 15 minutes of a meal); and controlled substances are counted and locked. Acetaminophen has a 4000 mg/day ceiling in healthy adults, and lower with liver disease or chronic alcohol use.

Error Prevention

Prevent errors by applying the rights and the three checks, avoiding do-not-use abbreviations (write out unit instead of U, and daily instead of OD), and using a leading zero for doses less than one (0.5 mg) but never a trailing zero (write 5 mg, not 5.0 mg, which can be misread as 50 mg). Double-check every calculation and every high-alert drug, and question any order that falls outside a safe range. If an error does occur, assess the client first, notify the prescriber, and complete an incident report honestly — transparency, not blame, is what protects future clients.

More Worked Calculations

Because dosage math carries heavy weight, practise the less obvious formats:

7. Liquid oral suspension. Order 375 mg; the bottle reads 125 mg per 5 mL. Amount = (375 / 125) x 5 mL = 3 x 5 = 15 mL. Remember to multiply by the quantity (5 mL), not just divide.

8. Units-based (heparin). Order 5000 units subcutaneously; the vial reads 10 000 units/mL. 5000 / 10 000 = 0.5 mL. Heparin is a high-alert drug, so this volume warrants an independent double-check.

9. Reconstituted powder. A vial reconstitutes to 250 mg/mL and 750 mg is ordered. 750 / 250 = 3 mL. Always note the concentration after reconstitution, not the powder weight.

10. Weight-based infusion converted to a rate. A 60 kg adult is ordered 2 mg/kg over 30 minutes: 2 x 60 = 120 mg total; if supplied as 120 mg in 100 mL on a pump, the rate is 100 mL / 0.5 h = 200 mL/h.

Estimating before you calculate is a powerful error check: if an answer means giving many tablets or an implausibly large volume, stop and recheck the unit conversion and the arithmetic before administering.