Dose Calculations Dr Hayder B Sahib Ph D

Dose Calculations Dr. Hayder B Sahib Ph. D, M. Sc. , D. Sc. , B. Sc Pharmacology

Quiz • How many (mg) each of Dextromethorphan HBr and Guaifenesin will be contained in each dose of the following prescription? • R • Dextromethorphan HBr 2 dg • Guaifenesin 2. 4 g • Cherry Syrup 0. 12 L • Sig. teaspoonful for cough • Q 2 • A vial of gentamicin contains 80 mg/ml. How many milliliter of injection should be administered to obtain 0. 02 g of drug?

Special consideration in dose determination for elderly patients: • 1 - Therapy is often initiated with a lower-than the usual adult dose. • 2 - Dose adjustment may be required based on therapeutic response • 3 - The patient physical condition may determined the drug dose and the rout of administration employed • 4 - The dose may be determined in part on the patients weight, BSA, health and disease state and pharmacokinetic factors

• 5 - Concomitant drug therapy may affect drug per dose effectiveness • 6 - A drugs dose may produce undesired adverse effect patient’s compliance • 7 - Complex dosage regimen of multiple drug therapy may affect patient’s compliance.

Dosage form applicable to pediatric and geriatric patients • Tablets capsules are preferred for the oral administration because of easy administration, ready identification, transportation and lower cost per dose but it is impossible or difficult for pediatric. • Liquid, suspension, drops are preferred for pediatric.

• Drug dosage based on Age • As mentioned earlier, the age of the patients is consider in the determination of drug dosage, pediatric dose is a fraction of adult dose • Young’s rules • Age /age +12 X Adult dose= dose of child

• Recently age does not consider a singularly valid criterion in the determination of a curate dosage for child. Currently when age is consider in determining dosage of potent therapeutic agents, it used generally in conjugated with another factor such as weight. • Cowling rule • Age at next birth day (years) X Adult dose / 24 = child dose • Frieds Rule for infant • = Age (month) X Adult dose /150 = Dose for infant • Clarks Rule based on weight • = weight (Ib)X Adult dose / 150 (average weight of adult in pound) = dose for infant

Example • An over the counter cough remedy contain 120 mg of dextromethorphan in a 60 ml bottle of product the label state 1. 5 teaspoonful for a child six years of age, How many mg of Dextromethorphan are contained in child dose? • Solution • 1 ½ teaspoonful =7. 5 ml • 120 mg 60 ml • X 7. 5 • X=15 mg of Dextromethorphan

Dose based on body weight • Express as mg/ kg. • Dose (mg) = patient weight (Kg) X drug dose (mg)/ 1 Kg • Example • The usual dose of chlorambucil is 150 µg/kg of body weight how many (mg) should be administered to a person weighing 154 Ib? • Solution • 150 µg/kg= 0. 15 mg/kg • Patient’s dose ( mg) = 154/2. 2 X 0. 15 =10. 5 mg chlorambucil

Dosing tables • Daily dose of 0. 5 mg /kg, how many 20 mg capsules of the drug product should be dispensed to a patient weighing 176 Ib if the dosage regimen calls for 15 weeks of therapy? • Solution • Patient dose (mg) = 176/2. 2 X 0. 5 =40 mg • Each capsul of 20 mg so it is 2 capsuls • 2 cap X 7 days X 15 Weeks =210 cap

Drug Dosage based on BSA • It can be used for two types of patients A) Cancer patients receiving chemotherapy • B) Pediatric patients. • The neonate who dosed based on weight with consideration of age and a variety of biochemical, physiological, functional, pathological and immunologic factor. • Please see the relationship of the BSA and weight in the text book

• Example • The useful equation to calculate the dose basing on BSA is • patients dose = patients BSA(m 2)/ 1. 73 (m 2)X drug dose (mg )

• Example • If the adult dose of drug is 100 mg, calculate the approximate dose for a child with a BSA of 0. 83 (m 2). Use the equation and the table in the text book • child dose= 0. 83 (m 2)/1. 73(m 2)X 100 mg = 47. 97 mg=48 mg • 0. 83(m 2) = 48% of adult dose this is from the table in the text book • 100 mg X 0. 48= 48 mg • Nomogram: • Equation used • Patient’s BSA (m 2) =√patient height (cm) X Patient’s weight (kg) / 3600 • Calculate the BSA for patient measuring 165 cm in height and weighing 65 kg. • BSA (m 2 ) = 165 cm x 65/3600= 1. 37 m 2

Chemotherapeutic drugs • Chemotherapy is drug that given to treat cancer either alone or in combination with other drugs or radiation. Almost all chemotherapeutic agents act by destroying cancer cells, the major category of therapeutic agents include • Alkylting agents • Antimetabolite • Plant alkaloid • Antitumour antibiotics • Steroid hormone • The combination of anticancer include two agents, three agents, four agent regimen • The reason for drug combination is to enhance the anti cancer activity by combining two, or three, or four agents acting as anti cancer of different mechanisms.