Pharmaceutics 1 Lecture 4: Dosage Calculation PDF

Lecture 4 Dr. Fatima Aloum  Posology Posology deals with the dose or quantity of drug which is administered to produce the therapeutic action. Dose of a drug is the amount of a medicinal preparation or of radiation to be administered at one time. Dosage Determination of the a...

Lecture 4 Dr. Fatima Aloum  Posology Posology deals with the dose or quantity of drug which is administered to produce the therapeutic action. Dose of a drug is the amount of a medicinal preparation or of radiation to be administered at one time. Dosage Determination of the amount, frequency and number of doses of medicine to be given to a particular patient. Dosage Calculation for Children Various rules for calculating infant’s and children’s dosages are used. All of these methods give approximate dosage. Children are sometimes more susceptible than adults to certain drugs. The following rules are used for calculation of doses for infants and children: 1.Young’s Rule (for Children 1 to 12 years) Child dose= (age in years / age (years) + 12) × adult dose. Example: A 7 years old pediatric patient is admitted to hospital. The dispenser is tasked with determining what dose of the medicine prescribed by the physician. If the adult dose is 100mg and the child weighs 40 kilogram, what dose should the child is administered by using young’s rule? Answer: Child dosage=7yrs/7yrs+12yrs*100mg= 7yrs/19yrs*100mg =37mg of medicine  2. Clark’s Rule (for Children below 2 years) Child dose= ( Weight in pounds / 150 ) × adult dose. Example 1: If an adult dose of medications calls for 30mg and the child weighs 30 Ibs, what is the child dose? Answer: Child dose=30Ibs/150Ibs*30mg=6mg of medicin  3. Fried’s Rule for infants up to 1 years Infant dose= ( Age in month/ 150 ) × adult dose. Example: A physician orders diphenhydramine 50mg oral administration every six hours PRN for pediatric patient who admitted to the hospital. Calculate the dose for a 4 yr old child using Fried’s rule? Answer: The child is four years old, therefore, 4years*12months/1year= 48 months, then; Pediatric dose= age (months)/150 × adult dose =48months*50mg/150months =2400mg/150=16mg of diphenhydramine.  4. Dilling’s Formula Child dose= ( Age in years/20 ) × adult dose. Example: A seven year old pediatric patient is admitted to hospital. The dispenser is tasked with determining what dose of the medicine prescribed by the physician. If the adult dose is 100mg and the child weighs forty kilogram, what dose should the child is administered by using young’s rule? Answer: Child dosage=7 yrs/20 yrs*100mg=35 mg of medicine 5. The square meter surface area method is more reliable method of relating dosage. Child's dose = Body surface area of child / Body surface area of adult x adult dose The average body surface area for an adult is taken as 1.73 (m² ). Child dose= Body surface area of child (m²) / 1.73 x adult dose Calculation of Doses for Adults Many physiological functions such as metabolic rate and kidney functions are proportional to body surface area. The dose for some drugs may be given as the amount of drug/ m² X body surface area. In such cases the dose for an individual may be calculated as follows: Individual’s dose = Amount of drug/ m² × Body surface area in m² Generally the drug doses are often given in mg/kg body weight and for individuals it may be calculated as follows: Individual’s dose = mg/kg × body weight in kg  Example 1. Calculate the dose of amoxicillin suspension in mLs for otitis media for a 1-yr-old child weighing 22 lb. The dose required is 40 mg/kg/day divided BID and the suspension comes in a concentration of 400 mg/5 mL Step 1. Convert pounds to kg: 22 lb × 1 kg/2.2 lb = 10 kg Step 2. Calculate the dose in mg: 10 kg × 40 mg/kg/day = 400 mg/day Step 3. Divide the dose by the 400 mg/day ÷ 2 (BID) = 200 frequency: mg/dose BID Step 4. Convert the mg dose to mL: 200 mg/dose ÷ 400 mg/5 mL = 2.5 mL BID  Example 2. Calculate the dose of ceftriaxone in mLs for meningitis for a 5-yr-old weighing 18 kg. The dose required is 100 mg/kg/day given IV once daily and the drug comes prediluted in a concentration of 40 mg/mL Step 1. Calculate the dose in mg: 18 kg × 100 mg/kg/day = 1800 mg/day Step 2. Divide the dose by the 1800 mg/day ÷ 1 (daily) = 1800 frequency: mg/dose Step 3. Convert the mg dose to mL: 1800 mg/dose ÷ 40 mg/mL = 45 mL once daily Many of drug substances are administered in very small (milligram) quantities, which require these substance to be weighed on sensitive balance. Due to the low dosage and potent nature of the drugs cannot be taken by the patient by himself, when the dose of drug is minute e.g. 0.05mg of drug, for some drugs. Solid dosage forms such as tablets and capsules must be prepared with fillers or diluents so that the size of dosage form is large enough to be handled easily. Dosage forms are needed to: i) provide a method for safe and convenient delivery of accurate dose of drug. ii) protect the drug from destructive effect of atmospheric oxygen or moisture i.e. from oxidation, hydrolysis and reduction e.g. the tablet may be given coating. iii) protect the drug from degradation by gastric acid after oral administration e.g. enteric coating may be given to tablets and capsules (enteric coating does not dissolve in acid but it will dissolve in alkaline pH). iv) obscure the bitter, salty or offensive taste or odour of a drug e.g. drug may be given in capsules or in the form of flavoured syrups. v) give the drugs (which are insoluble or unstable) in the form of liquid preparations e.g. suspensions. vi) give the drugs in the clear liquid forms e.g. solution. vii) obtain rate controlled and sustained release drug action e.g. controlled release tablets. viii) obtain drug action from topical application e.g. creams, transdermal patch etc. ix) obtain drug action by insertion into various body cavities e.g. suppositories. x) obtain the drug action by placing the drug directly into bloodstream or in body tissues e.g. injections. xi) obtain drug action through inhalation therapy e.g. inhalation aerosols. Additives in Dosage Forms The action of a dosage form and the form depends on the additives added. An additive is a non-drug component having a definite function to perform in a formulation. The additives generally used in formulations are classified as i) Vehicles : Vehicle is a term which can be used for any solid, semisolid or liquid material used in formulation for carrying a drug. These additives form the bulk of a dosage form and help to give a desired dosage form. Solvents are used to dissolve the drugs. These solvent may be aqueous or non-aqueous. For example: Base : These are semisolid vehicles into which drug substance may be incorporated in the formulation of ointment, paste, suppositories etc. Diluents: are the material added to increase the bulk or reduce the drug concentration. ii) Disintegrating agents The dosage form once ingested must disintegrate into smaller particles to give its drug contents to the body. For example in tablets, disintegrating agents are added to break the tablet into smaller particles. iii) Colouring agent to give a good, appropriate colour to the dosage form iv) Flavouring agent to give a suitable pleasant flavour to the preparation and is especially important in liquid dosage form v) Sweetening agents to give a sweet taste to the preparation vi) Manufacturing additives Packaging of dosage forms The proper packaging, labelling and storage of pharmaceutical products are essential for product stability and efficacious use. Different specifications are required for parenteral, non-parenteral, pressurized and bulk containers and for those made of glass, plastic and metal. Container is defined as a device which holds the pharmaceutical preparation. Desirable features of a container are: i) The container must be rigid enough to with stand wear and tear during normal handling and to prevent damage to the contents e.g. breaking of tablets. ii) The container material should not react with the contents. iii) The container must protect the contents from deterioration by environmental factors like oxygen and moisture. iv) The container must prevent loss of moisture from contents. v) The container must prevent leakage of contents. vi) The container must prevent entry of dirt, other chemical or microbiological contamination. vii) The container should be sterilizable without any change. viii) The container should give protection from light when required. ix) The container must not absorb anything i.e. medicament or additives from the contents. x) The container must not impart anything to the contents. xi) The container must be easy to label. xii) The container must be pharmaceutically elegant Types of Container: Containers are classified in two ways: a) According to their ability to protect the contents from external condition b) According to number they contain A) According to their ability to protect the contents from external condition: In this category they can be classified into six groups: i) Well closed containers This type of container protects the contents from external solids and from loss of the contents under ordinary conditions of handling, shipment, storage and distribution. ii) Tight container This type of containers protects the contents from contamination by external liquid, solid and vapour. It should prevent changes due to efflorescence, deliquescence or evaporation. iii) Hermetically sealed These containers will not allow air and other gases to pass through the container under normal condition of handling. These containers are sterile and used for holding sterile preparation. iv) Light resistant container These containers protects the contents from photochemical deterioration e.g. Amber glass and opaque plastic containers. v) Temper evident container These types of containers are fitted with a mechanism or device which reveals irreversibly the opening of container. vi) Child resistant These types of containers are designed in such a way that cannot be opened by children. b) According to number they contain: In this category, they can be classified into two groups i.e. i) Single dose container These containers are used to provide a single dose of the medicaments. These containers once opened cannot be resealed to maintain sterility e.g. ampoules. ii) Multi dose containers These containers allow withdrawal of successive doses without changing the strength or affecting the purity of the remaining contents. Multi dose containers are hermetically sealed e.g. vials.