A 12-year-old girl was prescribed promethazine 12.5mg bd for 3 days. Stock is 25mg tablet. Calculate the number of tablets she has to receive for a complete course.

Formula: Daily dose = dose per kg per day $\times$ body weight

Eli's body weight = 50 kg (12-year-old child)

Dose = 5 mg/kg/day

Daily dose = 5 mg/kg/day $\times 50$ kg = 250 mg/day

Formula: Concentration = $\frac{\text{mg per 5 ml}}{5 \text{ ml}}$

Concentration = $\frac{125 \text{ mg}}{5 \text{ ml}}$ = 25 mg/ml

Formula: Total drug = concentration $\times$ volume of bottle

Total drug = 25 mg/ml $\times 200$ ml = 5000 mg

Formula: Days = $\frac{\text{total drug}}{\text{daily dose}}$

Days = $\frac{5000 \text{ mg}}{250 \text{ mg/day}}$ = 20 days

The bottle will last $\mathbf{20}$ days.

20 days

Question 1 (b)

James is prescribed 50 mg oral qid for 5 days. Assume 50 mg tablets.

qid = 4 doses per day

Daily dose = 50 mg/dose $\times 4$ doses/day = 200 mg/day

Total dose = 200 mg/day $\times 5$ days = 1000 mg

Formula: Number of tablets = $\frac{\text{total dose}}{\text{mg per tablet}}$

Number of tablets = $\frac{1000 \text{ mg}}{50 \text{ mg/tablet}}$ = 20 tablets

Drew/James needs $\mathbf{20}$ tablets for the complete course.

20

Question 1 (c)

Assume: Patient receiving 500 ml serum IV over 10 hours. Drop factor = 20 drops/ml. Calculate drops per minute.

Time = 10 hours $\times 60$ min/hour = 600 min

Formula: ml/min = $\frac{\text{total volume}}{\text{total time}}$

ml/min = $\frac{500 \text{ ml}}{600 \text{ min}}$ = $\frac{5}{6}$ ml/min $\approx 0.833$ ml/min

Formula: gtt/min = ml/min $\times$ drop factor

gtt/min = $\frac{5}{6}$ ml/min $\times 20$ drops/ml = $\frac{100}{6}$ gtt/min $\approx 16.7$ gtt/min

Set the drip rate to $\mathbf{17}$ gtt/min (rounded).

17 gtt/min

Question 1 (d)

Assume: The 500 ml serum contains 1000 mg drug (e.g., 1 g), infused over 10 hours. Calculate mg/min.

Formula: Conc. = $\frac{\text{total drug}}{\text{volume}}$

Conc. = $\frac{1000 \text{ mg}}{500 \text{ ml}}$ = 2 mg/ml

From (c), ml/min = $\frac{5}{6}$ ml/min

mg/min = 2 mg/ml $\times \frac{5}{6}$ ml/min = $\frac{10}{6}$ mg/min $\approx 1.67$ mg/min

Lucy receives $\mathbf{1.67}$ mg/min.

1.67 mg/min

Question 1 (e)

From (b), daily tablets for James = $\frac{200 \text{ mg/day}}{50 \text{ mg/tablet}}$ = 4 tablets/day

Number of tablets taken every day: $\mathbf{4}$

4