13.2 Oral Medications, Liquid Suspensions & Solid Dose Calculations

Oral Medications, Liquid Suspensions & Solid Dose Calculations

Oral administration is the most common route for delivering pharmacological therapy in both antenatal and postnatal care. From managing gestational hypertension and obstetric infections to neonatal prophylaxis and analgesia, registered midwives must execute rapid, flawless calculations. In clinical practice, medications are rarely supplied in the exact single-dose quantity prescribed. The midwife must calculate the exact number of solid dosage units or liquid millilitres required to deliver the therapeutic dose safely.

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Solid Oral Dosage Forms: Tablets and Capsules

Solid oral medications are manufactured as tablets, caplets, or capsules. Calculating the number of units to administer utilizes the fundamental solid dosage formula:

$\text{Number of Tablets / Capsules} = \frac{\text{Dose Prescribed (What You Want)}}{\text{Stock Strength per Unit (What You Have)}}$

Clinical Example 1: Antihypertensive Therapy in Pre-eclampsia

• Prescription: A woman with pre-eclampsia is prescribed oral labetalol 400 mg twice daily.
• Available Stock: Labetalol 200 mg film-coated tablets.
• Calculation: $\text{Tablets to administer} = \frac{400\text{ mg}}{200\text{ mg}} = 2\text{ tablets}$

Clinical Example 2: Pre-Calculation Metric Conversion

• Prescription: A postnatal woman with severe mastitis is prescribed oral flucloxacillin 1 g four times daily.
• Available Stock: Flucloxacillin 500 mg capsules.
• Step 1 (Convert Units): The prescribed dose is in grams, but the stock is in milligrams. Convert grams to milligrams by multiplying by 1,000: $1\text{ g} \times 1,000 = 1,000\text{ mg}$
• Step 2 (Apply Formula): $\text{Capsules to administer} = \frac{1,000\text{ mg}}{500\text{ mg}} = 2\text{ capsules}$

Rules for Splitting and Crushing Solid Medications

A vital clinical competency tested in the CBT is recognizing which oral formulations may be split or crushed:

• Scored Tablets: Tablets manufactured with a central debossed line or groove are specifically designed to be snapped into equal halves (e.g. 0.5 tablet). If a calculation yields half a tablet (e.g. prescribed 5 mg, stock is 10 mg scored tablet), dividing the scored tablet is clinically acceptable.
• Unscored Tablets: Must never be cut or broken, as active pharmaceutical ingredients are not guaranteed to be evenly distributed throughout an unscored tablet matrix.
• Gelatin Capsules: Must never be opened or dissolved unless explicitly licensed for enteral suspension by the manufacturer.
• Modified-Release (MR / SR / XL / CR) Formulations: These tablets are engineered with polymer coats or matrix systems to release active drug slowly over 12 to 24 hours. Crushing or chewing an MR formulation causes dose dumping—the immediate, uncontrolled release of the entire 24-hour drug load into the maternal circulation. For example, crushing an oral modified-release nifedipine tablet results in sudden profound maternal hypotension, uterine hypoperfusion, and acute fetal bradycardia.
• Enteric-Coated (EC) Tablets: Coated with a gastro-resistant barrier designed to bypass the stomach and dissolve in the neutral pH of the duodenum. Crushing an enteric coating causes either severe gastric mucosal ulceration or destruction of acid-labile drugs by hydrochloric acid.

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Liquid Medications & Oral Suspensions: The Universal Formula

Liquid oral medications—including oral solutions, elixirs, syrups, and reconstituted powder suspensions—are expressed as a mass of drug dissolved in a specific volume of liquid (e.g., $125\text{ mg in } 5\text{ mL}$, or $250\text{ mg in } 5\text{ mL}$). To determine the volume to administer, midwives employ the universal liquid formula:

$\text{Volume to Administer (mL)} = \frac{\text{Dose Prescribed (What You Want)}}{\text{Stock Strength (What You Got)}} \times \text{Stock Volume}$

Where:

• Dose Prescribed: The exact quantity of drug ordered by the prescriber (numerator).
• Stock Strength: The mass of drug contained in the stock container (denominator).
• Stock Volume: The liquid volume that contains the stock mass (usually $1\text{ mL}$ or $5\text{ mL}$).

Step-by-Step Clinical Calculation Workflows

Example 1: Infant Paracetamol Suspension

• Prescription: An infant weighing 4 kg requires oral paracetamol 60 mg for post-immunisation pyrexia ($15\text{ mg/kg/dose}$).
• Available Stock: Paracetamol infant oral suspension $120\text{ mg in } 5\text{ mL}$.
• Step 1: Check units. Both are in milligrams (mg). No conversion needed.
• Step 2: Apply formula: $\text{Volume (mL)} = \frac{60\text{ mg}}{120\text{ mg}} \times 5\text{ mL} = 0.5 \times 5\text{ mL} = 2.5\text{ mL}$
• Sanity check: $60\text{ mg}$ is exactly half of $120\text{ mg}$; therefore, the volume must be exactly half of $5\text{ mL}$, which is $2.5\text{ mL}$.

Example 2: Antibiotic Suspension with Metric Conversion

• Prescription: A woman with a postpartum wound infection is prescribed oral cefalexin 0.75 g three times daily.
• Available Stock: Cefalexin oral suspension $250\text{ mg in } 5\text{ mL}$.
• Step 1 (Convert Units): The prescribed dose is in grams; the stock is in milligrams. Convert grams to milligrams: $0.75\text{ g} \times 1,000 = 750\text{ mg}$
• Step 2 (Apply Formula): $\text{Volume (mL)} = \frac{750\text{ mg}}{250\text{ mg}} \times 5\text{ mL} = 3 \times 5\text{ mL} = 15\text{ mL}$

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Paediatric and Neonatal Oral Dosing Based on Body Weight

Neonatal pharmacology requires heightened vigilance due to immature hepatic metabolism and renal clearance. Unlike adult dosing, which often relies on fixed standard quantities, neonatal and infant oral dosing is calculated strictly according to body weight in kilograms (kg).

The Critical Distinction: Per Dose vs. Per Day

A major pitfall in the CBT is failing to distinguish between two prescribing conventions:

1. $\text{mg/kg/dose}$: The calculated amount represents the quantity to give at each individual administration.
2. $\text{mg/kg/day divided into } X \text{ doses}$: The calculated amount represents the total 24-hour requirement, which must be divided by the number of daily administrations to find the single-dose volume.

                    WEIGHT-BASED DOSING WORKFLOW
                    
            Prescription: Dose based on body weight (mg/kg)
                                  │
                                  ▼
                   Is the prescription written as:
                                  │
         ┌────────────────────────┴────────────────────────┐
         ▼                                                 ▼
   "mg/kg/dose"                                     "mg/kg/day"
         │                                                 │
         ▼                                                 ▼
Calculate Single Dose:                            Calculate Total Daily Dose:
Weight (kg) × Dose (mg/kg)                        Weight (kg) × Dose (mg/kg/day)
         │                                                 │
         │                                                 ▼
         │                                        Divide by Number of Doses:
         │                                        Total Daily Dose ÷ Doses/Day
         │                                                 │
         └────────────────────────┬────────────────────────┘
                                  ▼
                        Single Dose (in mg)
                                  │
                                  ▼
                   Apply Liquid Volume Formula:
               (Dose ÷ Stock Strength) × Stock Volume
                                  │
                                  ▼
                     Measure in ENFit Syringe

Worked Example: Neonatal Fluconazole for Oral Candidiasis

• Clinical Scenario: A 7-day-old infant weighing $3.5\text{ kg}$ is diagnosed with severe oral thrush. The prescriber orders oral fluconazole $3\text{ mg/kg/day}$ as a single daily dose.
• Available Stock: Fluconazole oral suspension $50\text{ mg in } 5\text{ mL}$ ($10\text{ mg/mL}$).
• Step 1 (Calculate Dose): $\text{Dose (mg)} = 3.5\text{ kg} \times 3\text{ mg/kg} = 10.5\text{ mg}$
• Step 2 (Apply Liquid Formula): $\text{Volume (mL)} = \frac{10.5\text{ mg}}{50\text{ mg}} \times 5\text{ mL} = 0.21 \times 5\text{ mL} = 1.05\text{ mL}$
• Sanity check: $10\text{ mg/mL} \rightarrow 10.5\text{ mg}$ requires slightly more than $1\text{ mL}$. $1.05\text{ mL}$ is clinically precise.

Safe Administration: ENFit Enteral Syringes

Under NHS England patient safety alerts, all oral liquid medications must be measured and administered using dedicated purple enteral syringes (ENFit system). Enteral syringes cannot connect to intravenous ports, eliminating the fatal hazard of accidental intravenous injection of oral suspensions. When drawing up liquid, the measurement must be aligned precisely with the leading ring (flat stopper) of the syringe plunger, not the pointed tip or trailing flange.