Dosage Of Drugs (Posology)

Front 1

Therapeutic Dose

Back 1

Average dose required to produce the therapeutic effect, in adult male.

Front 2

Loading Dose

Back 2

Initial large dose to build up therapeutic blood level (LD = Vd * Css)

Vd: apparent volume of destribution

Css: steady state concentration

Front 3

Maintenance Dose

Back 3

daily dose required to maintain the therapeutic level (MD = Cl * Css * Tm)

Cl = clearance

Tm = time interval

Front 4

Minimal effective dose

Back 4

the lowest dose to produce therapeutic effect

Front 5

Maximal tolerated dose

Back 5

the largest dose without toxic effect

Front 6

Median Lethal dose

Back 6

the fatal dose in 50% of cases (kill 50% of animals)

Front 7

Uses of Median Lethal dose (LD50)

Back 7

- a measure of acute toxicity (the smaller, the more toxic)

- determines the therapeutic index and safety margin

- determines the GIT absorption (compare oral with IV)

- Clinical trials usually start at one tenth of LD50

Front 8

Therapeutic Index

Back 8

- the ratio of LD50 (the dose that produces toxicity in 50% of cases) to the ED50 ( the dose that produces an effective response in 50% of the cases)

- is a measure of drug's safety, because a larger value indicates a wide margin between doses that are effective and doses that are toxic.

Front 9

Therapeutic Window

Back 9

a more clinically relevant index of safety, describes the dosage range between the minimum effective therspeutic dose, and the minimum toxic dose.

Front 10

Factors affecting Action & Dosage of drugs:

Back 10

1- Biological Variation

2- Age

3- Weight & Surface Area

4- Sex

5- Route of Administration

6- Time of Administration

7- Cumulation & Zero order Kinetics

8- Psychological factors

9- Pathological state

10- Allergy

11- Idiosyncracy

12- Intolerance (Supersensitivity)

13- Tolerance & Dependance

14- Drug Interactions

Front 11

Factors affecting Action & Dosage of drugs:

2- Age:

Pediatrics differ from adult:

Back 11

- more body water, less fat, less plasma ptn, BBB more permeable, thin skin

- less Renal clearance e.g. aminoglycosides

- less HME activity Grey Baby syndrome

- behave differently to some drugs e.g. amphetamine produces sedation

- can tolerate some drugs than adults e.g. atropine & digitalis

Front 12

Factors affecting Action & Dosage of drugs:

2- Age:

Geriatrics differ from adult:

Back 12

- dec. GIT motility, blood flow & HCL secretion

- dec. lean body weight, water & relative inc. in fat.

- dec. HME activity & hepatic blood flow

- dec, GFR & tubular secretion

- inc. frequency of postural hypotension& dec. baroreceptor reflex

- inc. sensitivity to CNS depressants

Front 13

Factors affecting dosage and action of drugs:
4- Sex

Back 13

Females need smaller doses than males because more fat and estrogen inhibit HME.
Drugs specifically for females e.g. oxytocics and tocolytics.
Drugs avoided by females in certain periods:
- Menstruation Aspirin Bleeding
- Pregnancy Oxytocics Cathartics Teratogenic Abortion and fetal anomalies.
- Labour Morphine Barbiturates Neonatal asphyxia.
- Lactation: Purgatives antihistaminics anticoagulants tetracyclines.

Front 14

Factors affecting Action and Dosage of the drug:
8- Psychological factors

Back 14

Placebo effect response to psychological rather than pharmacological effects.
Placebo is an inert preparation used for therapeutic effects in psychotherapy and for evaluation of new drugs (Dynamic or Psychic Effect).

Front 15

Factors affecting Action and Dosage of the drug:
9- Pathological state

Back 15

Some drugs act ONLY in the presence of disease.
Pathology may cause supersensitivity.