calculating the dosage of drugs

1. DR.DHAVAL.S.PATEL

2. ■ Factor influencing the dose of a drug:
• Patient Factors:
1. Age
2. Weight
3. Body surface area
4. Sex
5. Severity of the disease under treatment
6. Status of renal or hepatic function
7. Species and race
8. Hereditary influence (genetics)
9. Food
10. Psychological factors
•Drugs factors:
1. Chemical nature of the drug
2. On repeated use of drugs (i) Tolerance
(ii) Resistance
3. Concurrent administration of the other drugs.
4. Timing, Rate &Route of administration of drugs

3. ■ Expression of Doses :
● For adults:
• Solid drugs in mg or gm/kg/day in daily divided doses
• Liquid in milliliter (ml) or liter
• Biological preparations(e.g. insulin) in I.U./kg/hrs
• Aerosol in puff/day
■ Dose calculations for pediatrics patients:
(a) on the basis of Age:
(i) Young’s formula:
Child’s dose=Adult dose×
𝐴𝑔𝑒 𝑖𝑛 𝑦𝑒𝑎𝑟𝑠
𝐴𝑔𝑒+12
(ii) Dilling ‘s formula:
Child ‘s dose=Adult dose×
𝐴𝑔𝑒 𝑖𝑛 𝑦𝑒𝑎𝑟𝑠
20
(b) On basis of Weight:
(i) Clark’s formula:
Child’s dose=Adult dose×
𝑊𝑒𝑖𝑔ℎ𝑡 (𝑘𝑔)
70
Child’s dose=Adult dose×
𝑊𝑒𝑖𝑔ℎ𝑡(𝑙𝑏𝑠)
150

4. (3)On the basis of BSA:
• Child’s dose: Adult ‘s dose×
𝐵𝑆𝐴(𝑚2)
1.73
► BSA is calculated by using :
Dubois formula: 𝐻𝑇(𝑐𝑚)0.725 × 𝐵𝑊(𝑘𝑔)0.425 × 0.007184
►Ready made charts/ slide rule nomograms for BSA:
Age Ideal
weight(kg)
BSA(𝑴𝟐) % of adult
dose
New born 3 0.22 12
3 months 6 0.33 18
1 yrs 10 0.45 28
5 ½ yrs 20 0.8 48
9 yrs 30 1.0 60
12 yrs 40 1.3 78
14 yrs 50 1.5 90

5. ► For elderly patients (> 60 yrs.) :
• No fixed rules or formulae for determining the dose in elderly
patients.
• Dose may be reduced to 2/3 of the adult dose .
▪ Should based on various factors:
(1) Types of drug being administered.
(2) Age related decline in renal or kidney
function.
► For patients of renal failure:
• Dose = Adult dose ×
𝐶𝑟𝑒𝑎𝑡𝑖𝑛𝑖𝑛𝑒 𝑐𝑙𝑒𝑎𝑟𝑎𝑛𝑐𝑒 𝑖𝑛 𝑟𝑒𝑛𝑎𝑙 𝑖𝑛𝑠𝑢𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦
𝐶𝑟𝑒𝑎𝑡𝑖𝑛𝑖𝑛𝑒 𝑐𝑙𝑒𝑎𝑟𝑎𝑛𝑐𝑒 𝑖𝑛 𝑛𝑜𝑟𝑚𝑎𝑙 𝑖𝑛𝑑𝑖𝑣𝑖𝑑𝑢𝑎𝑙
☻(Normal creatinine clearance for a 70 kg healthy man is 100-120 ml/min)

6. ►Strength of formulations and drug dilutions:
(1) Percentage: 1% solution means 1g of solute in 100 ml (10 mg/ml)
(2) W/V: for solids in liquid type of formulations
means number of grams of active substance in 100 ml of liquid product.
e.g. 5% dextrose contains 5 gm dextrose in 100 ml of infusion.
(3) W/W: for solids in solid type of preparations,
means number of grams of that constituent in 100 g of formulations,
e.g. 3% framycetin ointment contain 3g of framycetin in 100g of
- ointment.
(4) V/V: for liquids in liquid type of preparations,
means the volume of constituent (in ml) in 100 ml of total formulation.
e.g. 70% ethyl alcohol contains 70 ml ethyl alcohol in a total of 100 ml
-aqueous solution
(5) Proportion:
e.g. 1:1000 adrenaline hydrochloride injection means 1 gm (1000 mg) of
- active ingredient in 1000 ml (1gm adrenaline/ml)

7. ☻Assignment:
1. A 10 year old boy is prescribed syrup isoniazid for tuberculosis. if the duration
of treatment is 60 days ,how many bottles of isoniazid will be required?
Weight = 24 kg
Dose in children= 5 mg/kg/day.
Each bottle contains =100 mg/5 ml drug.
Bottle capacity =100 ml
2. A diabetic patient requires insulin at the rate of 0.1 U/kg/hr.
Set up insulin drip and calculate the rate of infusion.
Weight = 80kg
Insulin is available as 40 U/ml in 5 ml vial.
3. Mention the steps to prepare :
10 ml of 15% NaCl solution.
10 ml of isotonic saline solution.
4.Calculate the daily dose of amikacin for 75 years old female obese patient 90 kg
having S.creatinine 2.5 mg/dl suffering from renal parenchymal disease.
{ Cock croft –Gault equation:
Estimated Cr. Cl. (ml/min)=
(140−𝑎𝑔𝑒)×𝐵𝑊(𝑘𝑔)
72×𝑃𝑐𝑟(
𝑚𝑔
𝑑𝑙
)
. Multiply by 0.85 for women }