The document discusses important considerations for calculating drug dosages, including dose amount, dosage regimen, and factors that influence dosing schedules such as half-life, volume of distribution, clearance, and special conditions like renal failure or pediatric patients. It also provides examples of calculating loading and maintenance doses, intravenous fluid rates, and dosages for specific drugs and patient populations.

1. Calculation of drug dosage
(Posology)
Amit Kumar
Assistant professor
Pharmacology

2. • Dose- Require amount of the drug in weight, volume,
moles or international units that is necessary to
provide desire effect.
• Dosage regimen- Drug dose frequency What to give,
When to give, How much to give,

3. • Why dosage calculation is important ??
• To achieve desired therapeutic efficacy
• Variability in Pharmacokinetics and Pharmacodynamics
• In special condition
• To avoid ADR
• Interaction with other drugs

4. General consideration in dosing schedule
• Drugs with very short half life (in min) should be given by IV
infusion to maintain steady state plasma conc.
– Epinephrine 1-2 min
– Dopamine 1-2 min
– Dobutamine 1-2 min
– Oxytocin 1-2 min

5. • Drugs with short half life (30 to 2 hr) with high safety of margin
and first order kinetics, double dose can be given so that can be
administration every 6-8 hr.
– Cephalaxin 1hr
– Benzyl Penicillin 1 hr
– Paracetamol 2 hr

6. • Drug with half-life 4 – 12 hr, usually administration at every
half life interval
• If half-life 12 hr can be given 12 hourly
• If half-life 24 hr, half dose should be given every 12 hourly

7. • Drug with longer half life
– High volume of distribution
– Slow rate of clearance
– Cumulative in nature
• Digoxin 40 hr -640 L
• Desipramine 20-30 hr -60 L
• Diazepam 40 hr -50-70 L
• Digitoxin 168 hr -38 L
• Chloroquine 40 hr -130L

8. • These drug may take several days to achieve Steady state plasma
conc.
• In Non-emergency case Steady state plasma conc. attained in few
days
• In emergency case initial provide loading dose to achieve Steady
state plasma conc. In less time then maintenance doses

9. Loading dose
• It is large dose of the drugs to be given initially to provide
effective plasma conc. Rapidly.
• LD =
• Target Cp = Target conc. Of the drugs
• Vd = Volume of distribution
• F = Bioavailability
F =
Target Cp X Vd
F (between 0-1)
Quantity of drugs reaching in systemic circulation
Quantity of drug administered

10. Maintenance dose
• Dose required to maintain to steady plasma conc.
• Dosage rate =
• CL = Clearance
• Css = steady plasma conc
• F = Bioavailability
CL.Css
F

11. 2.Dose calculation in special condition
• Dose calculation for children
• Dose calculation for renal failure

12. 1. BODY SIZE/BODY WEIGHT
• Body size influences the concentration of the drug attained at
the site of action.
• Average adult dose refers to individuals of medium built.
• For obese or lean individuals dose may be calculated on body
weight (BW) basis:
individual dose = BW(Kg) x average adult dose
70

13. • The dose of a drug for children is often calculated from adult dose by
young’s, dilling’ and clark’s fomula
child dose = age (year) x adult dose (young’s formula)
age + 12
child dose = age (year) x adult dose (dilling’s formula)
20
child dose = Wt. of child(lb) x adult dose (Clark’s formula)
150
• Pediatric dose = child's age in months x Adult Dose (Fried's rule)
150

14. 2. Body surface area(BSA)
• Method is more reliable method for pediatric dose calculation. because
total body water, extra cellular fluid volume, & metabolic activity are
better correlated with BSA.
child dose = BSA(m2) of child x avg. adult dose
1.7
• Dose recommendation in terms of BSA are available only for
Anticancer.
(1.5 x wt in kg) +10= % adult dose
(0.7 x wt in lb) +10= % adult dose
Acc. BSA nomogram not
available

15. Creatinine clearance
(CL)Cr Men =
(CL)Cr Women = Male value X 0.85
Corrected Dose =
• Normal Creatinine clearance is taken as 100ml/min
(140-Age in year) X (weight in kg )
72 X serum creatinine in mg/100ml
Normal dose X pt. Creatinine clearance
Normal Creatinine clearance

16. Calculation of Intravenous Fluid
• 3 different methods – gtt/min
• METHOD I: Three-Step
• METHOD II: Two-Step
• METHOD III: One-Step

17. Calculation of Intravenous Fluid
• METHOD I: Three-Step
• Step 1: = ml/hr
• Step 2: = ml/min
• Step 3: = drop/min
Fluid ml
hrs to administer
ml per hr
60 minutes
ml/min x Drop/ml

18. Calculation of Intravenous Fluid
• METHOD II: Two-Step
• Step 1: = ml/hr
• Step 2: = gtt/min
Fluid ml
Hrs to administer
ml per hr x drops per ml
60 minutes

19. • METHOD III: One-Step
= gtt/min
Fluid ml x drop/mL
Hrs to administer x min/hr

20. • Drop factors:
• Macro drip – 15/ml
• Micro drip – 60/ml
• No. of drops/min =
• No of hours =
Fluids ml X Drop factor
No. of hours X 60
Fluids ml X Drop factor
no. of drops / min x 60

21. • Your drug order is for 1000mL of D5 to run for 8 hours. – Would you
used macrodrip or microdrip IV set? Calculate the drops per minute
(gtt/min) using the 2 step method.
Fluids ml X Drop factor
No. of hours X 60

22. • D5LR 1L for 12 hours. The drop factor is 15gtts/ml. Compute for the
no. of drops/min.
Fluids ml X Drop factor
No. of hours X 60

23. • D5LR 1L at 2o gtts/min. The IV set delivers at 15gtts/ml. Compute for
the no. of hours consumed.
Fluids ml X Drop factor
no. of drops / min x 60

24. • E.g.- A Patients weighing 50kg requires I.V. infusion of dopamine at the
rate of 5 mcg/kg/min. A 5 ml ampoule containing 40 mg/ml of dopamine
is diluted in 200ml of IV. Fluid
• How many drops/min of this I.V fluid will provide the required dose of
dopamine
(16 drop= 1ml)

25. 5 ml ampoule contains 40mg in 1ml
Thus it contains 40x5=200mg
Rate of infusion 5x50=250 mcg/min
250x60=15000mcg/min
=15mg/hr
200mg diluted in 200ml
Therefore 1mg in 1ml and 15mg in 15ml
Thus the require rate will be 15 ml in 1hr
So in 1min 15/60= ¼ ml to be given
Hence no. of drop /min 1/4x16=4drop/min

26. Measures of Mass
pico nano micro milli centi desi gm deca hecto kilo
1 kg = 1000 gm
(1 dekagram = 10 gm, 1 hectogram = 100gm)
1 gm = 1000 mg
1 mg = 1000 µg
1 µg = 1000 ng
1 ng = 1000 picogm
1 ounce = 30gm
1 pound = 454gm
1gm = 15 grain
1grain = 65mg

27. Measures of volume
1 Liter = 1000ml
HOUSEHOLD MEASURES
1 tea spoonful = 5ml
1 dessert spoonful = 10ml
1 table spoonful = 15ml
1 tea cup = 120 ml
1 glass = 240ml
1 wine glass = 60ml
1ml = 16drop

28. Percentage solution
Percentage concentration of a preparation is expressed in
three way
1. wt in wt
2. wt in volume
3. volume in volume

29. 1. Weight in Weight
Used for expressing percentage of a preparation, containing a solid in
solid,
e.g. Ointments, pastes, powders.
1% w/w means 1 part by wt of solid mixed with 99 pasts by wt of another
solid to make 1 part in 100 parts of finished product.
e.g. 1% of boric acid ointment would mean 1gm of boric acid present in
99gm paraffin wax to make 100gm of ointments.

30. 2. wt in volume
Used for expressing percentage of a preparation, containing a solid in
liquid,
e.g. Lotions.
1% w/volume means 1 gm of solute in 100 ml of solvent

31. 2. volume in volume
Used for expressing percentage of a preparation, containing a liquid in
liquid.
e.g. Emulsion.
1% volume/volume means 1 part by volume of a liquid( solute) dissolved
in a another liquid(solvent) to make 100 parts by volume of solution the
solution.
.e.g. 300ml of 5% solution of chloroform in a alcohol.

32. Note- sometime strength of the solution is mentioned in that case
percentage of solution can be calculated by multiplying strength by 100
e.g.-1 in 5 = 20% of solution.
1 in 10 = 10% of solution.

33. Convert the following
1. 1gm = ……..mg, ………µg.
2. 1tsf = ………..ml.
3. 1kg = ………..lbs.
4. 1mg = …….gm.
5. 1µg = …….gm.
6. 1ng = ……gm.
7. 1picogm = ………gm
Assignment

34. 1. Calculate the quantity of CHQ syrup to be prescribed if 8.3 mg/kg of
CHQ base is to be administered for a child weighing 2o kg.
• (Available as 100 mg of base per 10 ml oral suspension)
2. How many milligram of Lignocaine are there in 30 ml of 2% Lignocaine.

35. 3. Calculate the dose of Paracetamol for child aged 2 years. The Adult dose
of Paracetamol is 500 mg every 8 hourly.
• (Available as a suspension in the conc. of 250mg/5 ml)
4. If you wish to prescribe Metronidazole for a child weighing 10kg (0.46
m2) suffering from acute amoebiasis. We can calculate by using BSA
method.
• Adult dose (2400mg OD)

36. 5. You have decided to give IV infusion of quinine to a 50 kg adult suffering
from cerebral malaria.
• Calculate the dose of drug.
• Calculate the volume of drug
• Calculate the rate of infusion
•
• Note- Dose 10 mg/kg
• ampoule contain 300mg/ml (diluted in 500 ml D5)
• Rate 20 drop/min/ml

37. 6. A patients to the casualty with sudden onset of chest pain. On examination he has
cold and clammy skin and is sweating. The body weight is 60 kg. ECG was
suggestive of MI. an iv drip of dopamine has to be set for this patient to treat
cardiogenic shock.
• Calculate the dose of drug.
• Calculate the volume of drug
• Calculate the rate of infusion
• Note- Dose 5 mcg/kg/min
• Amppoules of 200mg/5ml, dopamine is diluted in 200ml of IV fluid.
• Rate 20 drop/min/ml

38. 7. Your drug order is for 500mL of RL to run for 10 hours. – Would you
used micro drip IV set, Calculate the drops per minute (gtt/min) using
the 3 step method.
8. RL 2L for 15 hours. The drop factor is 15gtts/ml. Compute for the no. of
drops/min.
9. Target plasma conc. Of theophylline IV Route is 10mg/L and clearance
2.8L/h/70kg calculate maintenance dose of theophylline.
10. If desire plasma conc. Of digoxin for therapeutic effect is 1-2 mcg/L
and Vd is 640 then calculate loading dose in mg.

39. 7. A Patients weighing 40kg requires I.V. infusion of dopamine at the rate
of 5mcg/kg/min. A 5 ml ampoule containing 40 mg/ml of dopamine is
diluted in 500ml of IV. Fluid
• How many drops/min of this I.V fluid will provide the required dose of
dopamine
(20 drop= 1ml)
8. The measured CLcr in a patients with renal insufficiency is 12ml/min,
calculate the dose of gentamicin.