Pharmacology - Basic Principles

Original Editor - Mason Trauger Top Contributors - Mason Trauger, Stacy Schiurring, Kim Jackson, Jess Bell and Kirenga Bamurange Liliane

Introduction to Pharmacology and Pharmacotherapy

Pharmacology and pharmacotherapy are closely related fields. Pharmacology is the scientific study of medications and the principles of drug action on living organisms. Pharmacotherapy refers to the use of medications to treat diseases, manage symptoms, or alter the progression of health conditions in a clinical setting. It encompasses all aspects of drug therapy, from selecting the appropriate medication and determining the right dose to monitoring its effects on the body. Pharmacotherapy relies on a thorough understanding of pharmacology to ensure safe and effective drug use.

In the field of pharmacology, drug is a broad term referring to any chemical substance that, when administered to a living organism, produces a biological effect. A medication is a specific type of drug that is used to treat, cure, prevent, or diagnose a disease, or to promote well-being. Essentially, medication is a subset of the broader category of drugs, but the terms are often used interchangeably.

What the Body Does to a Drug and a Drug Does to the Body

Pharmacokinetics is the study of how medications move through the body. It encompasses four key processes: absorption, distribution, metabolism, and excretion. Understanding pharmacokinetics is essential for determining the most suitable dosage, route, and frequency of medication administration to ensure that therapeutic effects (the intended beneficial effects of the medication) are achieved, while minimising unwanted side effects (the unwanted effects of the medication).

Absorption refers to how a drug enters the bloodstream and is influenced by factors such as the medication’s form or route of administration (e.g., pill/tablet, injection, topical cream), the site of administration, and individual patient characteristics, such as activity level, gastric pH, and food intake.

Once absorbed, the drug is distributed to various tissues via the bloodstream. This process is affected by lipid solubility, protein binding, and regional blood flow. Distribution also depends on the drug’s ability to cross protective barriers, such as the blood-brain barrier or placenta.

Metabolism is the process by which the body breaks down and converts medications into different substances, known as metabolites. This typically occurs in the liver, where enzymes play a key role in altering the drug's chemical structure. The metabolites produced can be either inactive or active, and in some cases, the metabolites may have a different effect than the original medication. The speed and efficiency of metabolism vary among individuals based on genetics, age, liver function, and interactions with other substances.

Excretion removes the drug and its metabolites from the body, mainly through the kidneys in urine, though other routes include bile, sweat, and exhalation. The half-life of a medication (the time it takes for the concentration in the bloodstream to decrease by half) plays a key role in determining how long it stays in the body and, therefore, in determining dosing intervals. Factors such as kidney function and hydration status can affect the rate of excretion, which influences the medication’s effectiveness and the risk of side effects.

The dose-response curve identifies the range of therapeutic effectiveness from the threshold dose to the ceiling effect of a drug.

Pharmacodynamics is the study of how medications interact with the body to produce their intended effects. It focuses on a drug’s mechanism of action, particularly how it binds to target molecules (e.g., receptors, enzymes, or ion channels) to either activate or inhibit natural biological processes. These interactions underpin a medication’s therapeutic effects. A central concept in pharmacodynamics is the dose-response relationship, which describes how the strength of a drug’s effect changes with varying concentrations. This helps identify the minimum effective dose and the point beyond which higher doses offer no added benefit, known as the ceiling effect. Drug dosing principles are often based off of a range related to the dose-response relationship curve and bioavailability. Bioavailability refers to the extent and rate at which the active drug ingredient, or its therapeutic metabolite, enters the systemic circulation and ultimately reaches a specific site of action. Potency refers to how much of a drug is needed to achieve a given effect, while efficacy denotes the maximum effect a drug can produce, regardless of dose. Medications may act as agonists (stimulating a response), antagonists (blocking a response), partial agonists (producing a limited effect), or inverse agonists (producing the opposite effect). Another vital concept is the therapeutic index, which indicates a drug’s safety margin by comparing the effective dose to the toxic dose. Drugs with a narrow therapeutic index require close monitoring due to a higher risk of adverse effects. Pharmacodynamics also explores issues such as tolerance and desensitisation, where the body’s response to a drug diminishes with repeated use; as well as drug interactions, where one medication may enhance or reduce the effects of another.

The relationship between pharmacodynamics and pharmacokinetics is essential for effective drug therapy: while pharmacokinetics determines how much of a drug reaches its target site and for how long, pharmacodynamics explains what the drug does once it gets there. Essentially, pharmacokinetics is what the body does to a drug, and pharmacodynamics is what a drug does to the body. Together, these concepts guide clinicians in developing safe and effective treatment plans that optimise therapeutic benefit and minimise harm.

Common Routes of Drug Administration

Medications can be administered through various routes. The chosen route depends on factors such as the drug’s formulation, desired speed of action, and the patient’s condition and needs.

Enteral routes of administration utilise the gastrointestinal (GI) system, and include oral, sublingual, buccal, and rectal routes. The oral route (PO), where patients swallow tablets, capsules, or liquids, is the most common and convenient; however, it requires the drug to pass through the digestive system, which can delay absorption and be affected by food or gastric pH. This is known as the first-pass effect (also known as first-pass metabolism or presystemic metabolism), which is a phenomenon where the concentration of a drug is reduced due to metabolism in the liver before it reaches the systemic circulation.

Sublingual and buccal routes involve placing the drug under the tongue (sublingual) or against the cheek (buccal), allowing for faster absorption directly into the bloodstream. These routes avoid the first-pass effect.

Parenteral routes bypass the GI system and typically involve injecting the medication directly into the body. This allows for rapid absorption into the bloodstream. Parenteral routes include intravenous (IV), intramuscular (IM), and subcutaneous (SC) injections; IV is the fastest route as it delivers the drug directly into circulation.

Topical administration applies medication directly to the skin or mucous membranes for local effects, while transdermal patches offer systemic effects through slow, sustained absorption. Inhalation delivers drugs directly to the lungs. This method is useful for respiratory conditions, and it provides rapid systemic absorption. Rectal and vaginal routes may be used when oral administration is not possible, offering local or systemic effects.

Pharmacology and Rehabilitation Practice

Though medications are not typically prescribed by physiotherapists/physical therapists, knowledge of this field is pertinent to the care of patients and clients. Knowledge of drugs, drug metabolism, the impact of patient diagnoses and their effects on various body systems enhances a clinician's ability to evaluate a patient, monitor responses to activity, provide education, and engage in interprofessional discussions with pharmacists, physicians, etc.

Moreover, medications can significantly influence both the safety and effectiveness of therapy. Patients in rehabilitation often take medications to manage conditions such as pain, inflammation, muscle spasms, or neurological symptoms, each of which can affect physical function and the patient’s response to treatment. By being informed about how these medications work, their potential side effects, and possible interactions, rehabilitation professionals can adjust therapy plans to maximise benefit and minimise risk. Medication side effects may impact motivation, cognition, or energy levels, all of which are critical to successful rehabilitation outcomes.

Additionally, rehabilitation professionals are uniquely positioned to provide ongoing patient education across multiple sessions. This includes explaining how medications may affect recovery, helping patients recognise potential side effects, and emphasising the importance of medication adherence. If prescribed medications are not taken as directed, patients may miss out on crucial therapeutic benefits and protective effects, potentially hindering their overall progress in rehabilitation.

Please review the following optional articles if you would like more information on the role of rehabilitation professionals and medication:

• Behavioural Approaches to Pain Management
• Chronic Pain
• Communication in Chronic Pain Conditions
• Medication and Older People
• Pain Neuroscience Education
• Pain Neuroscience Education for Children
• Pain Neuroscience Education for Low Back Pain
• Patient Education in Pain Management
• Prescribing Rights in the UK

Resources

This video provides an excellent overview of the pharmacology and pharmacotherapy concepts discussed on this reading page.

This video simply explains drug interactions using clinical examples.

Finally, this video discusses polypharmacy and techniques for managing and preventing it.