Factors Affecting Posology

Posology factors

Posology is a branch of pharmacological science dealing with the amount of dose administered to patients. The word posology is derived from the Greek words ‘posos‘ meaning “how much” and ‘logos‘ meaning “science.” Posology can also be defined as the science of the amount of a drug given to the patient to produce desired pharmacological and therapeutic effect. The amount of dose of drug varies from person to person, depending on their age, body weight, sex, and other factors. If the dose of a drug is not determined properly then some medications may turn toxic or can even lead to the death of the patient. For example, Digitalis is a drug used to improve heart function but can also lead to death if consumed in excess. To achieve the therapeutic effect for each drug, the dose should be administered accurately, taking into consideration the factors impacting the dose of a drug. 

Factors affecting Posology

1. Age

Age is one of the significant factors which holds a major impact on drug dose administration. Different age groups are prescribed different doses of drugs to avoid drug toxicity in the body. The immature hepatic and renal functions in infants are the primary reason that newborns are easily affected by some medications, which in turn restrict the detoxification and elimination of drugs from the body. The drug then gets accumulated in tissues to a toxic level. Elderly people (geriatric) are also at high risk of developing toxicity of drugs in the body tissues because of their weak organ functioning. The inability to detoxify and eliminate the drug from the body increases the probability of drug accumulation in tissues. Some drugs like hypnotics may produce confusion in elderly people due to their sedative effect. Dosage for children is categorized into three age groups, 2-6 years of age, 6-12 years of age, and children above 12 years. For children who are 2 years old or younger, one must consult a physician. Dose calculation formulas for pediatrics were used earlier. Today, these formulas are not in use as age is not a single factor affecting posology. Formulas that were used earlier for pediatric dose calculation are shown below-

Posology dosage age chart

Note: The value of 150 in Fried’s rule was an estimate of the age (12.5 years or 150 months) of an individual who would normally receive an adult dose, and the number 150 in Clark’s rule was an estimate of the weight of an individual who likewise would receive an adult dose.

 2. Sex

Drugs’ actions on men and women remain the same, but the output can be different. Both men and women respond differently to the action of drugs. For example, men’s response to sedatives, such as morphine and barbiturates, is much quicker than women’s. Women take longer time to act on sedatives than men. Women undergo lactation, pregnancy, and menstruation due to which extra caution should be taken while prescribing medicines during these phases. Drugs stimulating uterine smooth muscles, such as purgatives, anti-malarials, and ergot alkaloids, must be avoided during pregnancy. Apart from these, some other drugs are also prohibited during pregnancy like barbiturates, opioids, alcohol-containing drugs, and narcotic drugs because they can cause harm to the fetus through the placenta.  

3. Body weight

The dose of drugs administered according to body weight is always prescribed in milligrams per kilogram (mg/kg) of body weight. This method works only for people ranging from 50-100 kg of body weight. For people who are obese, malnourished, or young children, this technique of dose administration will not work, and thus, it will require a dose adjustment. Thin, malnourished, and obese people must receive dosage according to their body weight. The body weight medication technique is preferred over the age method among children. The dose calculation formula based on body weight is shown in the chart below.

Posology dose chart on body weight basis

4. Body surface area

This method for dose calculation is far better as compared to body weight in terms of accuracy. Nomogram is used to calculate the surface area of human beings with various scales like height, weight, etc. A person’s height and weight are connected by drawing a straight line. The points where these intersect are considered to be the surface area. The formula used for the calculation of body surface area is-

BSA (m2) = BW(kg)0.425  x  Height (cm)0.725  x  0.007184

5. Time of administration

The time at which a drug is consumed plays an important role, which decides the effectiveness of the drug. Drugs get absorbed rapidly in an empty stomach while the absorption reduces with the presence of food. Every drug acts differently as the time of drug administration depends on drug interactions with food. Therefore, a drug, that is much more effective, if taken before the meal will not show its therapeutic action if taken during or after the meal.

6. Route of administration

The route through which a drug is being administered also highly affects the effectiveness and the therapeutic action of drugs. For example, the dose administered through an intravenous route is slightly lower than the dose given orally. This is because intravenous drugs are injected directly into the blood through veins, which increases the chances of toxicity in the body as they hold a rapid mode of action. Oral drugs take longer time than intravenous drugs to be absorbed completely into the body, hence their dose is higher than that of intravenous.

Intravenous drug delivery

7. Environmental factors

The amount of drugs prescribed is also affected by the environment. Sedatives are drugs consumed at night to induce sleep and treat insomnia (sleeping disorder), but if taken in the daytime, the dose required will be much higher than taken at night. Similarly, stimulant drugs are mostly prescribed to be consumed in the daytime, but if it has to be consumed at night, then the dose amount should be higher. Other than day and night, the weather also affects the dose amount. For example, alcohol is more tolerant in winter than in summer.

8. Accumulation

Some drugs get accumulated in tissues of the body, inducing toxic effects, which can be harmful to the body and can even lead to death. Digitalis and emetine are the most common examples of such drugs. This toxic effect is induced when a drug is not able to eliminate completely from the body. The inability to detoxify and eliminate the drugs can increase toxicity in the tissues of the body. This accumulation of toxicity can be the result of repeated consumption of drugs like digitalis and heavy metals.

9. Presence of disease (pathological state)

Drug-disease interaction can be harmful to patients. Drug-disease interaction, despite inducing therapeutic effects, shows hazardous responses to the body and can even cause death. For example, streptomycin is eliminated via the kidney, and people suffering from renal failure are at high risk of toxicity. The absorption of aspirin decreases in people suffering from achlorhydria (gastrointestinal disorder). Metabolism of some drugs like morphine decreases in people with liver cirrhosis disorder.

Renal failure

10. Additive effect

This effect is basically a drug-drug interaction that is done intentionally to induce an effective and therapeutic effect. In other words, if the pharmacological action of two drugs is equal to the total of their separate actions then it is known as the additive effect. Aspirin and paracetamol, ephedrine and aminophylline are a perfect fit for such an effect.

Effect A + Effect B=Effect AB 

11. Synergism

Synergism is a phenomenon of inducing an enhanced therapeutic effect by mixing two drugs when the single drug is unable to produce the desired effect. It is also categorized under drug-drug interactions and is also called potentiation. Sulfamethoxazole and Trimethoprim are examples of synergism effects.

Effect AB > Effect A + Effect B

12. Antagonism

Antagonism method is highly effective in neutralizing poisoning effects. This means, when an action of a drug is opposed by the other on the same pharmacological system then antagonism occurs. For example, adrenaline and acetylcholine adrenaline and acetylcholine, when administered together, neutralize each other as a result of the antagonism effect as the former is a vasoconstrictor, and the latter is the vasodilator.

13. Idiosyncrasy

Idiosyncrasy can be best defined as the abnormal response of a drug to a particular patient different from its pharmacological effect. It can also be termed as another name for “Allergy.” For example, Penicillin can cause rashes on the skin in some patients. Aspirin may cause gastrointestinal hemorrhage even at modest doses.

Penicillin caused rashes

14. Tachyphylaxis

It is an irreversible phenomenon of developing acute tolerance against a drug in which cell receptors get blocked and the whole pharmacological action of the drug is reduced. It happens when a drug is taken repeatedly after short intervals. This phenomenon cannot be reversed even if the dose of the drug is increased. For example, repeated doses of Ephedrine in the treatment of bronchial asthma may induce very less response.

15. Metabolic disturbances

Metabolic disturbances like water-electrolyte balance change, acid-base equilibrium alterations, and body temperature may impact the effect of medicines. For example, Salicylates only lower high body temperatures, and they do not possess antipyretic (an agent that reduces fever) properties. 

16. Tolerance

Tolerance is the ability of a drug to withstand the effects of a drug. Tolerance is developed by repeated use of drugs for a longer period of time, and it, therefore, requires, a high dose of the drug to induce a therapeutic effect of the drug. For example, smokers can tolerate nicotine, and alcoholics can tolerate alcohol in large quantities. Tolerance is of three subtypes-

  • Natural tolerance – It is mostly seen in rabbits and black race people (Africans) as they are more tolerant to atropine drugs.
  • Acquired tolerance – It is defined as the tolerance which is developed by repeated use of a particular drug.
  • Cross tolerance – Cross tolerance can be best defined as the tolerance developed by pharmacologically related drugs. For example, alcohol-consuming people are more tolerant to sedatives.

17. Genetic diseases

People lack some enzymes due to genetic defects, and for such people, some drugs are restricted to be prescribed. Patients who lack ‘Glucose-6-phosphate dehydrogenase enzyme’ must not be prescribed “primaquine,” an antimalarial drug, that will cause hemolysis.

Glucose-6-phosphate dehydrogenase enzyme

18. Drug dependence

Drug dependence is of two types-

  • Physical dependence- In this, patients rely completely on drugs in order to function. This may cause withdrawal syndrome when a drug is abruptly stopped. Tea and nicotine are examples of physical drug dependence.
  • Psychological dependence- It is the drug desire and tendency to relapse when the drug is not available. Drug dependence


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