Drug dose calculation and relevant practice resources
Drug dose calculation and relevant practice resources
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Resource links online
Drug calculations - Ggeeky Medics
Drug Calculation Cases - Ggeeky Medics
Pharmaceutical Calculations Made Easy!
Drug Calculations: How to Use the Universal Formula
Drug Calculations - Flinders University
Pharmacy Calculations - StatPearls
Drug Dosage Calculations - George Brown College
Half-life of a drug (t1/2) calculation
The half-life of a drug (t1/2) is the time required for the drug's concentration in the body to decrease by exactly 50%. It can be calculated using two main formulas depending on the data available:
Volume of Distribution
Vd of drug C was determined to be 20 L. What would be the expected drug plasma concentration immediately after an IV dose of 2 mg is given to a 70 kg male patient?
A. 0.01 mirogram/dL
B. 1 mircrogram/ dL
C. 0.1 microgram/ dL
D. 10 microgram/ dL
Formula: Vd= dose/ Cp
Cp = Do/ Vd
Do = 2000ug
Vd = 20L = 200dL (10dL = 1L)
Cp = 2000/200 = 10 ug/dL
Answer: D. 10 microgram/ dL
Directions for questions 1-30: Each question, statement, or incomplete statement in this section can be correctly answered or completed by one of the suggested answers or phrases. Choose the best answer.
1. If a vitamin solution contains 0.5 mg of fluoride ion in each milliliter, then how many milligrams of fluoride ion would be provided by a dropper that delivers 0.6 mL?
(A) 0.3 mg
(B) 0.1 mg
(C) 1 mg
(D) 0.83 mg
We know, C1V1=C2V2
In this problem, V1=0.5mg; C1=1.0 milliliter; C2=?; V2=0.6mL
C2=C1V1/V2
C2=0.5x1/0.6
2. How many chloramphenicol capsules, each containing 250 mg, are needed to provide 25 mg per kg per day for 7 days for a person weighing 200 lb?
(A) 90 capsules
(B) 64 capsules,
(C) 13 capsules
(D) 25 capsules
We know, C1V1=C2V2
In this problem,
3. If 3.17 kg of a drug is used to make 50,000 tablets, how many milligrams will 30 tablets contain?
(A) 1.9 mg
(B) 1900 mg
(C) 0.0019 mg
(D) 3.2 mg
We know, C1V1=C2V2
In this problem,
4. A capsule contains a gr of ephedrine sulfate, 1½ gr of theophylline, and 16 1/16 gr of phenobar-bital. What is the total mass of the active ingre-dients in milligrams?
(A) 20 mg
(B) 8 mg
(C) 28 mg
(D) 4 mg
We know, C1V1=C2V2
In this problem,
5. If 1 fluid ounce of a cough syrup contains 10 gr of sodium citrate, how many milligrams are contained in 10 mL?
(A) 650 mg
(B) 65 mg
(C) 217 mg
(D) 20 mg
We know, C1V1=C2V2
In this problem,
6. How many capsules, each containing grof phenobarbital, can be manufactured if a bot-tle containing 2 avoirdupois ounces of pheno barbital is available?
(A) 771 capsules
(B) 350 capsules
(C) 3500 capsules
(D) 1250 capsules
We know, C1V1=C2V2
In this problem,
7. Using the formula for calamine lotion, determine the amount of calamine (in grams) necessary to prepare 240 mL of lotion.
Calamine -80 g
Zinc oxide -80 g
Glycerin -20 mL
Bentonite magma -250 ml
Calcium hydroxide -sufficient quantity to make 1000 mL topical solution
(A) 19.2 g
(B 140 g
(C) 100 g
(D) 24 g
We know, C1V1=C2V2
In this problem,
8. From the following formula, calculate the amount of white wax required to make 1 lb of cold cream. Determine the mass in grams.
Cetyl esters wax
12.5 parts
White wax
12.0 parts
Mineral oil
56.0 parts
Sodium borate
0.5 parts
Purified water
19.0 parts
(A) 56.75 g
(B) 254.24 g
(C) 54.48 g
(D) 86.26 g
We know, C1V1=C2V2
In this problem,
9. How many grams of aspirin should be used to prepare 1.255 kg of the powder?
ASA
Phenacetin
6 parts
Caffeine
3 parts
1 part
(A) 125 g
(B) 750 g
(C) 175 g
(D) 360 g
We know, C1V1=C2V2
In this problem,
Essential Formulas & QC Parameters used in lab
💧 1. Fundamental Unit Conversions
Understanding baseline volume and mass equivalencies is critical for accurate solution preparation in analytical labs:
• 1000 mL = 1000 cm3 = 1 dm3 = 1 L
• Density = Mass / Volume (Mass = Volume x Density)
• Water reference (at 4°C): 1 kg = 1 L = 1 dm3 | 1 g = 1 mL = 1 cm3
⚖️ 2. Stock Solutions, Dilutions & PPM Conversions
Preparing stock solutions accurately prevents errors downstream. Use standard dilution rules to adjust working concentrations:
• Dilution Formula: C1V1 = C2V2 = C3V3
• 1000 mg / 1000 mL = 1 mg / 1 mL = 1 g / 1 L = 1000 ppm
• 1% = 10,000 ppm = 1 g / 100 mL
• 1 ppm = 0.001 mg / mL = 1 ug / mL
🧪 3. Volumetric Analysis: Molarity & Normality
Molarity measures concentration by moles, whereas Normality accounts for reactive capacity using equivalent weights:
• Molarity (M): Moles of solute / Liters of solution
* Formula: M = (Mass / Molar Mass) / Liters of solution
* Dilution / Mixing: M1V1 = M2V2
• Normality (N): Gram Equivalent Weight / Liters of solution
* Equivalent Mass = Molar Mass / n (where n = valency factor or number of reacting equivalents)
* Example: For H2SO4, n = 2; for HCl, n = 1 (where N = M)
• Core Relationship: N = n x M (Normality is always equal to or greater than Molarity)
• Titration Equation: (M1 x V1) / n1 = (M2 x V2) / n2
📈 4. Detailed HPLC Assay Formula
HPLC assays quantify active ingredient potency by benchmarking sample peak response against a certified reference standard:
Assay % = (Area S / Area R) x (R eq wt / dil) x (dil / S eq wt) x (Avg wt / LC) x Assay % x factor x 100
• Area S: Peak area of Sample
• Area R: Peak area of Reference Standard
• R eq wt: Reference equivalent weight = (50 / 0.96) x (1 / Assay %)
• S eq wt: Sample equivalent weight = (Avg wt / LC) x 50
• Avg wt: Average weight of dosage units
• LC: Label Claim
• Factor: Base/salt conversion factor (always less than 1)
📉 5. Gravimetric QC Tests
Gravimetric analysis monitors mass changes. Key weights: W0 (empty container), W1 (container + sample before test), W2 (container + sample after test).
Wi = W1 - W0 (Initial sample weight)
Wf = W2 - W0 (Final residue weight)
• Loss on Drying (LOD) / Friability (Weight Loss):
Percentage Loss = ((Wi - Wf) / Wi) x 100
• Residue on Ignition / Sulfated Ash (Weight Retained):
Percentage Retained = (Wf / Wi) x 100
🔍 6. Calibration & Weight Variation
• Percentage Deviation = ((Actual - Theoretical) / Theoretical) x 100
(Note: If the factor calculated is positive (+ve), apply a negative (-ve) correction factor, and vice-versa).
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