Oral & Parenteral Dosages

How to read this guide. Values are labeled EXACT, nursing convention (≈), or illustrative/hypothetical. Any unlabeled value is a general teaching convention — never a universal limit or a real product fact. Always defer to the actual label, the written order, and your facility's policy.

3.1 Overview: two big families

This section covers how to turn an ordered dose into a measurable, administrable amount:

• Oral (enteral) — tablets, capsules, and oral liquids, given by mouth or feeding tube.
• Parenteral — injections: IM (intramuscular), SubQ (subcutaneous), ID (intradermal), and insulin.

Both use the same math (Section 2). What changes is what you can physically measure — a tablet can only be split so far, a syringe only reads so finely, and a site only holds so much volume.

PART A — ORAL SOLIDS

3.2 Tablets and capsules

Key idea: the ordered amount and the tablet strength must be in the same unit; then divide.

Formula: tabs = Desired ÷ Have (× 1 tablet) DA: ordered mg × (1 tab / strength mg) = tabs

Worked example — whole tablets

Order: 500 mg PO. Available: 250 mg per tablet.

500 mg × (1 tab / 250 mg) = 2 tabs

Formula: 500 ÷ 250 = 2 tabs. Ratio: 250 mg : 1 tab = 500 mg : X → X = 2 tabs. Answer: 2 tabs.

Worked example — with a conversion

Order: 1 g PO. Available: 500 mg tablets.

1 g × (1000 mg / 1 g) × (1 tab / 500 mg) = 2 tabs

Worked example — capsules (never split)

Order: 100 mg. Available: 50 mg capsules.

100 mg × (1 cap / 50 mg) = 2 capsules

Capsules contain a set amount and cannot be split (the contents are a measured dose). If the math yields a fraction of a capsule, that's your signal to stop and clarify.

3.3 Scored tablets, half-tablets, and when splitting is OK

• A scored tablet has a groove that the manufacturer placed there to indicate it may be split into (usually two) equal halves.
• A half-tablet dose arises when the ordered dose is half the available strength.

Worked example — half tablet

Order: 12.5 mg. Available: 25 mg scored tablets.

12.5 mg × (1 tab / 25 mg) = 0.5 tab  →  half tablet

Rules for splitting (safe practice):

1. Only split tablets that are scored (and only when policy/product labeling permits). Splitting an unscored tablet gives uneven, unreliable halves.
2. Do not split capsules, or enteric-coated / extended-release / controlled-release / sustained-release products (see 3.4) unless the labeling or a pharmacist explicitly authorizes it.
3. A "half" that isn't exactly half — if the strength isn't evenly divisible, the dose is not practical; clarify.
4. When in doubt, ask the pharmacist whether a specific product may be split. Not every scored tablet is authorized for splitting in every facility.

Do NOT assume every scored tablet should be split. Scoring permits splitting; it does not obligate it, and some scored products (e.g., certain narrow-therapeutic-index drugs) may still require whole-tablet dosing per policy.

3.4 Enteric-coated and modified-release products — do not crush

Tablet coatings and matrixes exist to control where and how fast a drug releases. Crushing, chewing, or splitting them defeats that design and can be dangerous.

Clinical pearl: crushing an ER/CR/SR product can release the entire dose immediately — turning a 24-hour opioid, antihypertensive, or cardiac dose into an acute overdose. When a patient can't swallow pills, the correct action is not to crush — it is to ask the pharmacist for a liquid or other appropriate formulation.

3.5 Recognizing impractical tablet doses

A calculation that produces a dose you cannot physically give is your cue to stop, do not "round to nearest," and notify/clarify.

Worked example — impractical dose:

Order: 100 mg. Available: 75 mg tablets (unscored).

100 mg × (1 tab / 75 mg) = 1.33 tabs

1.33 tablets cannot be measured (the tablet is unscored and can't be cut into thirds). The correct answer is not "give 1 tablet" — it is clarify the order with the provider/pharmacist (e.g., change the dose or the available strength).

Red flags for impractical doses:

• A fraction of a tablet/capsule that cannot be achieved (e.g., 0.33, 0.67 of an unscored tablet).
• A result requiring more than a reasonable number of tablets (e.g., 8 tablets of a low-strength product — flag it; a higher-strength product or different route may be intended).
• Any result that forces you to crush/split a modified-release or enteric product.

PART B — ORAL LIQUIDS

3.6 Concentrations: mg/mL and mg/5 mL

Oral liquids are labeled with a concentration. Learn to read both common formats:

• mg/mL — e.g., "20 mg/mL" (20 mg in 1 mL).
• mg/5 mL — e.g., "250 mg/5 mL" (250 mg in 5 mL — a teaspoon-sized reference).

DA setup:

ordered mg × (mL / available mg) = mL

Worked example — mg/5 mL

Order: 375 mg. Available: 250 mg/5 mL.

375 mg × (5 mL / 250 mg) = 7.5 mL

Formula: 375/250 × 5 = 7.5 mL. Ratio: 250 mg : 5 mL = 375 mg : X → 250X = 1875 → X = 7.5 mL. Answer: 7.5 mL.

Worked example — mg/mL

Order: 40 mg. Available: 20 mg/mL.

40 mg × (1 mL / 20 mg) = 2 mL

Worked example — with a conversion

Order: 0.5 g. Available: 125 mg/5 mL.

0.5 g × (1000 mg / 1 g) × (5 mL / 125 mg) = 20 mL

(0.5 g = 500 mg; 500 × 5 ÷ 125 = 20 mL.)

3.7 Device selection and measurable precision

The device you measure with determines how precisely you can deliver — and therefore how you round.

Rules:

1. Match the device to the volume and precision needed. A 0.4 mL dose requires an oral syringe, not a cup. A 15 mL dose is appropriate for a calibrated cup (or a 10 mL syringe drawn appropriately).
2. Round to what the device can measure. If the syringe reads to 0.1 mL, round to the nearest tenth (e.g., 7.54 → 7.5 mL). If a 1 mL syringe reads to 0.01 mL, you may round to the nearest hundredth.
3. There is no universal "minimum measurable volume." The smallest reliable volume is whatever the specific device's graduations can measure. State the device, then state the rounding. Do not cite a single universal minimum (e.g., "never measure below 0.5 mL") as fact — a 1 mL oral syringe with 0.01 mL graduations can measure far below that.
4. Never measure oral liquid with a parenteral syringe (and never administer oral liquid via a parenteral syringe with a needle). Use a dedicated oral syringe (typically no Luer/needle tip).

Worked example — device-matched rounding:

Order: 12.5 mg. Available: 50 mg/5 mL. Oral syringe calibrated to 0.1 mL.

12.5 mg × (5 mL / 50 mg) = 1.25 mL  →  round to 1.3 mL (nearest tenth, syringe reads 0.1 mL)

Raw = 1.25 mL; with a 0.1-mL syringe you'd deliver 1.2 or 1.3 mL — state the device and the rounding you applied.

PART C — PARENTERAL (INJECTIONS)

3.8 Routes at a glance

3.9 Syringe types and reading graduations

Reading graduations — the "count the spaces" method

The value of each small line = (difference between two labeled marks) ÷ (number of spaces between them).

3-mL syringe example: labeled marks are 0, ½, 1, 1½, 2 … mL. Between "1" and "1½" (0.5 mL apart) there are 5 spaces, so each line = 0.5 ÷ 5 = 0.1 mL. To draw 1.6 mL, count 1.5 + one line.

Tuberculin example: labeled marks are 0, .1, .2, … 1 mL. Between ".1" and ".2" (0.1 mL apart) there are 10 spaces, so each line = 0.01 mL. To draw 0.43 mL, go to 0.4 + three lines.

3.10 Worked parenteral examples

IM injection

Order: 50 mg IM. Available: 100 mg/2 mL. How many mL? (3-mL syringe)

50 mg × (2 mL / 100 mg) = 1 mL

Formula: 50/100 × 2 = 1 mL. Answer: 1 mL (measurable on a 3-mL syringe, reads to 0.1 mL).

SubQ injection

Order: 40 mg subcutaneously. Available: 40 mg/0.4 mL.

40 mg × (0.4 mL / 40 mg) = 0.4 mL

Answer: 0.4 mL — measure with a 1-mL (tuberculin or low-dose) syringe, not a 3-mL syringe, for precision.

ID injection

Order: purified protein derivative (PPD) 0.1 mL intradermally.

No dose math needed — the order is already in mL.

Answer: 0.1 mL — use a tuberculin syringe (0.01 mL graduations) to measure exactly 0.1 mL.

Insulin (U-100)

Concept: U-100 insulin = 100 units per 1 mL. Insulin is ordered in units, drawn in units on an insulin syringe, and never measured in a standard mL syringe.

Order: 20 units regular insulin subcutaneously. Available: U-100 insulin.

20 units × (1 mL / 100 units) = 0.2 mL

But you do not measure 0.2 mL in a 3-mL syringe. You draw 20 units on a U-100 insulin syringe (which is calibrated directly in units). Answer: 20 units.

Critical: only insulin syringes for insulin; only U-100 syringes for U-100 insulin. Using a mL syringe for insulin (or the wrong insulin syringe) causes exactly the kind of 10× error the "Do Not Use" rules are designed to prevent.

Weight-based parenteral (multi-step)

Order: 0.5 mg/kg SubQ. Weight: 40 kg. Available: 10 mg/mL.

40 kg × (0.5 mg / 1 kg) × (1 mL / 10 mg) = 2 mL

Answer: 2 mL. Clinical judgment: 2 mL exceeds the typical SubQ volume (≈1 mL) — flag it and verify whether the dose should be split into two sites or given by a different route/concentration.

3.11 Injection-volume limits — educational ranges, NOT universal limits

The "maximum volume per site" figures below are typical educational ranges used to teach safe injection practice. They are not universal limits and vary with muscle size, patient age, the specific drug, and facility policy. Always follow your facility's policy and the manufacturer's directions.

Treat these as study heuristics, not absolute rules. On an exam, if a question asks the "usual maximum," answer from the range your textbook/lecturer taught — and know that the clinical answer is always "per facility policy and product labeling."

3.12 Common mistakes

1. Splitting an unscored or modified-release tablet — uneven dose or dose-dumping.
2. Crushing ER/EC/CR/SR for a patient who "can't swallow" — dangerous; get a liquid instead.
3. Using a 3-mL syringe for a 0.4 mL or 0.05 mL dose — imprecise; use a 1-mL/tuberculin syringe.
4. Measuring insulin in mL instead of units (or with a non-insulin syringe).
5. Treating "1.33 tablets" as "1 tablet" by rounding — instead of flagging the impractical dose.
6. Rounding a liquid to a precision the device can't deliver (e.g., stating 1.27 mL on a 0.1-mL syringe).
7. Overfilling a site beyond its typical volume without verifying.
8. Forgetting to convert g→mg or lb→kg before the injection math.

3.13 Safety implications

• Modified-release products are high-risk when altered: crushing can cause fatal dose-dumping.
• Insulin is a high-alert drug: the wrong syringe or a units/mL mix-up is a classic 10×–100× error.
• Device precision is a safety boundary: you can only be as accurate as the syringe you hold.
• Impractical doses must be escalated, not forced: "rounding up" a tablet fraction is a med error waiting to happen.
• Site volume limits protect tissue integrity and drug absorption — exceeding them can cause pain, leakage, and unreliable absorption.

3.14 Memory aids

• Crush rule: "EC/ER/CR/SR — don't you dare" (never crush/split without authorization).
• Scored = splittable (sometimes); unscored = unsplittable.
• Syringe match: "Small dose, small syringe" (0.4 mL → 1-mL syringe; 1 mL+ → 3-mL syringe).
• Insulin: "Units on an insulin syringe — never mL, never a plain syringe."
• Device precision: "Round to what the barrel can show."

3.15 Exam traps

1. A question that asks you to give "half a tablet" of an unscored or ER product — the correct answer is clarify/do not split, not "half."
2. A patient with dysphagia + an ER capsule — the safe answer is request a liquid formulation, not "crush it."
3. An insulin order offered with a 3-mL syringe as the answer — reject it; you need a U-100 insulin syringe.
4. A "how many mL" answer like 1.27 mL when the syringe reads only 0.1 mL — round to the device's precision (and say so).
5. A distractor that rounds an impractical tablet dose (1.33 → 1) instead of flagging it.
6. A question testing ID vs SubQ vs IM volume ranges — recall the educational ranges but recognize the wording "usual/typical," not "universal."

3.16 Clinical pearls

• Always read the label twice: strength, form (scored? ER? liquid?), and concentration.
• For oral liquids, pick the device first, then round — precision follows the device.
• Insulin: match syringe type to insulin type (U-100 → U-100 syringe); verify with a second nurse per policy.
• Keep a "small dose → small syringe" reflex: 1-mL/tuberculin syringe for anything under ~1 mL needing precision.
• When the math produces an unmeasurable or impractical result, that's a clarification trigger, not a rounding exercise.

3.17 Check Yourself

Answers below — attempt each before looking.

1. Order: 750 mg. Available: 250 mg scored tablets. How many tablets?
2. Order: 0.6 g. Available: 300 mg capsules. How many capsules?
3. Order: 62.5 mg. Available: 125 mg scored tablets. How many tablets? Is this dose practical?
4. Order: 250 mg. Available: 200 mg/5 mL. How many mL? (round to nearest tenth)
5. Order: 0.5 mg. Available: 1 mg/2 mL. How many mL? Which syringe is most appropriate?
6. Order: 35 units U-100 insulin SubQ. How many units do you draw, and with which syringe?
7. Order: 150 mg. Available: 100 mg tablets (unscored). What is the appropriate action?
8. Order: 3 mg/kg. Weight: 50 kg. Available: 50 mg/mL. How many mL?

Answers

1. 3 tabs (750/250).
2. 0.6 g = 600 mg → 2 capsules (600/300).
3. 62.5/125 = 0.5 tab (half) — practical only if the scored tablet is authorized for splitting.
4. 250 × 5 ÷ 200 = 6.25 → 6.3 mL (nearest tenth).
5. 0.5 × 2 ÷ 1 = 1 mL — a 1-mL (tuberculin/low-dose) syringe is most appropriate for precision.
6. 35 units, drawn on a U-100 insulin syringe.
7. 150/100 = 1.5 tabs, but the tablet is unscored → clarify with provider/pharmacist; do not "give 1.5" or guess.
8. 50 kg × 3 = 150 mg; 150 × 1 ÷ 50 = 3 mL. DA: 50 kg × (3 mg/1 kg) × (1 mL/50 mg) = 150/50 = 3 mL.

3.18 Rounding rules (oral & parenteral)

Key references

• The Joint Commission — Official "Do Not Use" List (safe-order context).
• Institute for Safe Medication Practices (ISMP) — error-prevention guidance on insulin syringe use, dose expressions, and oral-syringe safety.
• Manufacturer product labeling and facility policy for all formulation-specific facts (scoring, crushability, concentrations, injection volumes).

Concentrations and injection-volume figures in this guide are illustrative or educational conventions, not real product facts or universal limits.