Factors Affecting Dose Selection (2023)

Factors Affecting Dose Selection (1)


Typically, a range is used to express the dose. While the maximum dose, also known as the higher limit of the dose, is the quantity of the pharmacological substance that can be tolerated by the average person, the minimum dose, also known as the lower limit of the dose, is crucial for eliciting the targeted therapeutic response.

These doses are recommended for the prescriber's guidance. The maximum dose, which, if exceeded, may have undesirable consequences on the patient, is of great concern to the pharmacist. The prescriber will determine the exact amount of a medication based on the patient's age, sex, symptoms, medication history, and other characteristics like tolerance, individuality, and mode of administration.

A measured quantity of a drug or medicine that is meant to be taken all at once is known as a dosage. Example: The infection can be cured with a single dose of penicillin.

A drug's prescribed dose determines whether patients experience maximum effectiveness, toxicity that could result in death, or no impact at all. For disorders and/or medications that can reduce severe morbidity or extend life, dosage is very crucial.

Where a medicine has a high risk of mortality or serious morbidity, dosage is also crucial. We feel there are numerous instances where more precise dosing could be advantageous to patients, thus it is important to think about how to order drug-disease targets.

Factors affecting dose selection are as follows.

(Video) Posology | Factors Affecting Dose of drug | Introduction of Posology | Hindi |Pharmacy Lecture


01. Age

Children typically need lower amounts than adults. Children's doses can be calculated using either Young's formula (based on age) or Clark's formula (based on weight), although the formula based on body surface area is more dependable.


02. Body weight

The typical pharmacological doses are described usually for an adult weighing 70 kg. The ratio between the amount of drug delivered and the size of the body determines the drug concentration at the site of action. For patients who are excessively thin or obese, the dose calculations must be made based on body weight.


03. Sex

The dosing alters significantly between male and female due to certain pharmacokinetics differences among them .This is particularly crucial when doing sex hormone therapy. Adult females often need lower doses than adult males since they have more body fat.


04. Time and frequency of administration

The key element affecting how frequently a medicine is administered is the biological half-life of the drug, or the amount of time needed for the blood level to fall to 50% of the initial peak level. For instance, following the initial dose of 2 g of sulphadiazine, 1 g of the medication must be administered every 4 hours if its biological half-life is 4 hours. However, in some cases, such as when reserpine is used as a tranquiliser, the biological half-life of a drug has no bearing on how frequently it is administered.


05. Body surface area

The surface area of the body and numerous physiological systems are tightly connected. The nomogram, which includes scales for height, weight, and surface area, can be used to determine the human surface area.


06. Route of administration

In general, the order of the route of administration affects how quickly a medicine is absorbed: intramuscular, intravenous, subcutaneous, and oral. As a result, a drug's intravenous (intravenous) dose is often lower than its intramuscular (intramuscular), subcutaneous (subcutaneous), or oral (oral) dose.

For instance, the following are the ergotamine dosages for various routes.

  • 2 to 5 mg orally

  • 1 mg intramuscularly (about half the oral dose)

    (Video) Posology | Factors affecting Dose of a drug (Pharmaceutics-1)

  • 0.25 mg intravenously, which is roughly equal to 1/8 of the oral and IM doses.


07. Emotional factors

Due to the fact that women experience emotions differently than men, the dosage of some medications must be changed to produce the desired results.


08. Environmental factor

The effect of medicine and outcome depends on environmental conditions like temperature, humidity and coolness. Example: stimulating medicine is effective in daytime.Hypnotic medicines are effective in night time due to darkness of night which has a soothing effect.


09. Morbid condition

A lesser dose is recommended if the organs that carry out biotransformation or excretion are ill. For instance, phenobarbitone, which is primarily excreted by the kidneys, should be administered in smaller doses in cases of renal insufficiency, and morphine should be administered in smaller doses in cases of patients with liver problems (morphine is mainly inactivated in liver).

Although aspirin lowers body temperatures in patients who are feverish, it has no effect on normal body temperature. Black water fever is more frequently precipitated by quinine when falciparum malaria is present than not.


10. Synergism

When two drugs work together, for example, cocaine and adrenaline, the combined effect is larger than the effects of the individual substances added together algebraically.


11. Antagonism

When two medications are used that have opposite effects, such as when amphetamine is administered to partially reverse the sedation brought on by anticonvulsant doses of phenobarbital and ephedrine is given to reverse the hypotension brought on by spinal anesthesia.


12. Additive effect

The combined impact of two or more medications, such as carbachol and acetylcholine, when administered together is the algebraic total of each drug's separate effects.


13. Idiosyncrasy

Idiosyncratic drug reactions are described as side effects that do not occur at any dose in the majority of patients, primarily develop unexpectedly in vulnerable people exclusively, and cannot be explained by the known mechanisms of action of the offending agent. According on the definition chosen, these reactions may occur more frequently and account for up to 10% of all adverse drug reactions.In allergic patients, penicillin may cause anaphylactic shock (sudden drop in blood pressure).

(Video) POSOLOGY: Factors Affecting Dose

However, not in healthy patients.The Indian Pharmacopoeia lists the dosages for frequently prescribed medications. While morphine typically calms the central nervous system, some people, particularly women, may experience excitement when using it.


14. Tachyphylaxis

Tachyphylaxis is the gradual loss of response to a dose following repeated administration of a pharmacologically or physiologically active agent; the symptoms may also occur during antidepressant therapy. Although the exact incidence of the condition is unknown, it may be as high as 33% when treating depression pharmacologically.


15. Accumulation

The drug accumulation ratio (Rac), which is used in the field of pharmacokinetics, measures how much a drug accumulates under steady state circumstances (i.e., following repeated administration) in comparison to a single dosage. The amount of drug accumulation in the body increases with increasing value. A Rac value of 1 indicates no buildup.


16. Addiction

Drug addiction, also known as substance use disorder, is an illness that affects a person's brain and behavior and causes them to lose control over whether or not they take drugs or medications, whether they are legal or not. Alcohol, marijuana, and nicotine are examples of substances that are regarded as drugs. If you are hooked, you might use the drug even when it is harmful.

ADRs and side effects are influenced by a variety of circumstances. Some of these variables can be altered, such as smoking or alcohol consumption, while others, such as age, the existence of other diseases, or hereditary factors, cannot.

Healthcare providers are better able to select the best suitable medication for a given patient when they are aware of the various implications these factors have on dose decisions. Additionally, it aids healthcare providers in providing patients with the finest guidance.

Here are some frequently asked questions on factors affecting

01. Is dose and dosage different?

Yes, a dose is a set quantity of medication administered all at once. Contrarily, the dosage is the recommended way to take the drug: a specified quantity, number, and frequency of doses spread over a specific amount of time.

02. What is Young's rule?

[Age / (Age + 12)] x Recommended Adult Dose = Pediatric Dose

(Video) Posology | Factors affecting Dose of a drug | Various Formulas for Dose Calculation | Pharmaceutics

03. Can Young's rule be used in all ages of pediatrics?

Yes,Young's rule can be applied to 12 yrs of age and When the patient's weight is unknown, Young's Rule might be used to rapidly approach the situation.


  1. What are the Factors Affecting the Dose and Action of Drugs | Article Shared By Kalpana | www.preservearticles.com

  2. Drug accumulation ratio | www.wikipedia.org


Factors Affecting Dose Selection (2)

Dr. N L Swathi

She’s a Pharm.D student at Sri Venkateswara College of Pharmacy. She’s interested in pathophysiology and research, and is working as an intern at PharMSkooL to hone her skills.

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