Oral Dosage: Tablets and Capsules

Every shift, nurses administer oral medications more often than any other route. Tablets and capsules account for the majority of those doses. Getting the count right is essential — too few tablets means the patient is underdosed, and too many could cause toxicity. The math itself is straightforward once you understand the D/H x Q formula, but the real-world details of scored versus unscored tablets, capsules that cannot be split, and multi-tablet doses all add clinical judgment to the calculation.

The Formula for Tablets and Capsules

For oral solids, the quantity $Q$ is always 1 tablet (or 1 capsule). This simplifies the formula:

$\text{Tablets} = \frac{D}{H} \times Q = \frac{D}{H} \times 1 = \frac{D}{H}$

Where:

• $D$ = Desired dose from the prescriber’s order (in mg, mcg, or g)
• $H$ = Have — the strength printed on the medication label (same unit as D)

Before plugging into the formula, always verify that $D$ and $H$ share the same unit. If the order reads 0.5 g and the label reads 500 mg, convert 0.5 g to 500 mg first.

Scored vs. Unscored Tablets

A scored tablet has a line etched across its surface that allows it to be broken cleanly into halves (or sometimes quarters). An unscored tablet has a smooth surface and cannot be split reliably — breaking it may produce uneven pieces with unpredictable dosing.

Rules:

• If your calculation yields a half-tablet (e.g., 1.5), you may administer it only if the tablet is scored.
• If the tablet is unscored and your answer is not a whole number, contact the prescriber or pharmacist for an alternative strength or form.
• Capsules can never be split. If the calculation produces a fractional capsule, the dose cannot be given in that form.

Step-by-Step Method

1. Read the order. Identify the drug name, dose ($D$), route (PO), and frequency.
2. Read the label. Identify the drug name, strength ($H$), and whether the tablet is scored.
3. Verify units match. Convert if necessary (g to mg, mg to mcg, etc.).
4. Apply the formula. Divide $D$ by $H$.
5. Evaluate the answer. Check whether the result is clinically reasonable and physically possible to administer.

Worked Examples

Example 1: Lisinopril (Simple Whole-Tablet Dose)

Order: Lisinopril 20 mg PO daily Available: Lisinopril 10 mg tablets (scored)

$\text{Tablets} = \frac{20 \text{ mg}}{10 \text{ mg}} = 2 \text{ tablets}$

Answer: Administer 2 tablets once daily.

Reasonableness check: Lisinopril is commonly prescribed at 10 to 40 mg daily. Two 10 mg tablets giving 20 mg total is well within the normal adult range. This is a reasonable answer.

Example 2: Furosemide (Half-Tablet Dose)

Order: Furosemide 60 mg PO daily Available: Furosemide 40 mg scored tablets

$\text{Tablets} = \frac{60 \text{ mg}}{40 \text{ mg}} = 1.5 \text{ tablets}$

Answer: Administer 1.5 tablets — one whole tablet plus one half of a scored tablet.

Reasonableness check: Furosemide doses typically range from 20 to 80 mg for adults. A 60 mg dose is within the expected range. Because the tablet is scored, splitting is appropriate. If the tablets were unscored, you would need to contact the prescriber.

Example 3: Metformin (Unit Conversion Required)

Order: Metformin 1 g PO BID Available: Metformin 500 mg tablets

The order is in grams but the label is in milligrams. Convert first:

$1 \text{ g} = 1{,}000 \text{ mg}$

Now apply the formula:

$\text{Tablets} = \frac{1{,}000 \text{ mg}}{500 \text{ mg}} = 2 \text{ tablets}$

Answer: Administer 2 tablets per dose, twice daily.

Reasonableness check: Metformin doses range from 500 to 2,550 mg per day. Taking 1,000 mg (two tablets) twice daily equals 2,000 mg per day, which is a standard dose. Without the unit conversion, you might have set up $\frac{1}{500}$, which gives 0.002 tablets — an obviously incorrect result.

When to Question the Answer

Your calculation should raise a flag if any of the following occur:

Rounding Rules for Tablets

• Scored tablets: Round to the nearest 0.5 tablet (half-tablet). If the calculation gives 1.5, administer 1.5. If it gives 1.3, you cannot administer 1.3 of a tablet — contact the prescriber.
• Unscored tablets: The answer must be a whole number. If it is not, the available strength does not match the ordered dose.
• Capsules: The answer must be a whole number. Capsules are never split or opened unless verified as safe by consulting an authoritative drug reference guide, the manufacturer’s instructions, or a pharmacist.

Common Mistakes

1. Mixing up D and H. The desired dose (from the order) always goes in the numerator. The available strength (from the label) goes in the denominator. Reversing them inverts your answer — if the correct answer is 2 tablets, you would get 0.5 instead.

2. Forgetting unit conversion. Orders in grams with labels in milligrams (or vice versa) will produce wildly incorrect results. Always convert before calculating. A 0.25 g order with a 250 mg tablet should yield 1 tablet, not 0.001.

3. Splitting unscored tablets or capsules. Even if the math says 0.5, the physical medication may not allow it. Unscored tablets crumble unevenly, and capsules contain premeasured powder or beads. Splitting either one is clinically inappropriate without pharmacist guidance.

4. Accepting unreasonable answers without questioning. If you calculate 5 tablets for a single dose of an antihypertensive, something is wrong. Always compare your answer to the typical dose range in a drug reference.

5. Confusing the drug name. Two drugs can have similar names (e.g., Hydroxyzine vs. Hydralazine). Always verify the drug name on the label matches the drug name on the order before calculating.

Practice Problems

Test your understanding with these clinical scenarios. Click to reveal each answer.

Key Takeaways

• For tablets and capsules, the D/H x Q formula simplifies to $\frac{D}{H}$ because $Q = 1$
• Always confirm that $D$ and $H$ use the same unit before dividing
• Scored tablets can be split into halves; unscored tablets and capsules cannot
• If your answer exceeds 3 tablets or produces a fraction that cannot be administered, verify the order and contact the prescriber
• Every calculation should end with a reasonableness check — compare the dose to the typical adult range in a drug reference

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