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HLT 214 - Substance Use and Abuse - Textbook

Chapter 3 : Understanding the Science of Addiction - Basic Pharmacology 

Overview

Pharmacology is the study of drugs, their uses and how they affect organisms.  In order to understand the science of addiction, first you will be introduced to the language of basic pharmacology.

Drug Administration

The dose is the quantity of drug, or dosage form, administered to a subject at a given time. This can be expressed as the weight of drug (e.g. 250 mg), volume of drug solution (e.g. 10 mL, 2 drops), the number of dosage forms (e.g. 1 capsule, 1 suppository, 1 tablet) or some other quantity (e.g. 2 puffs for inhalers). A therapeutic dose is amount of medication required to produce the desired outcome. A subtherapeutic dose is. a dose of a drug that does not achieve a particular therapeutic effect. Although this is generally not desired, drugs intended for one purpose may be administered in subtherapeutic doses to achieve a different effect.  For example, the usual dose of aspirin for relief of pain in an adult is 300-600 milligrams, whereas a low-dose aspirin such as 75 to 150 milligrams (mg) may be prescribed to prevent heart attacks.  

The dosage regimen is the frequency at which the drug doses are given. Examples include 2 250 mg capsules twice a day, one tablet three times a day, one 5mg injection every four weeks.

The total daily dose is calculated from the dose and the number of times per day the dose is taken. For a person prescribed 2 250 mg capsules twice daily, the total daily dose is 1000mg.

The dosage form is the physical form of a dose of drug. Common dosage forms include tablets, capsules, creams, ointments, suppositories, drops, aerosols, and patches. 

The optimal dosage is the dosage that gives the desired effect with minimum side effects. There are many factors taken into consideration when deciding a dose of drug - including age of the patient, weight, sex, ethnicity, liver and kidney function and whether the patient smokes. A consideration of drug interaction with other medicines may also affect the drug dose.

The route of administration is the way the dosage form is given. Common routes of administration include1

  1. Enteral Routes
    1. Sublingual (buccal)-given beneath the tongue or retained in the cheek pouch and are absorbed from these regions into the local circulation.  Examples of drugs administered by this route include antianginal like nitrites and nitrates. 
    2. Oral- the most common route. The passage of drug from the gut into the blood is influenced by a number of factors. Many different medications are taken orally (by mouth). They come as solid tablets, capsules, chewable tablets or lozenges to be swallowed whole or sucked on, or as drinkable liquids such as drops, syrups or solutions. Per os abbreviated to P.O. is sometimes used as a direction for medication to be taken orally. 
    3. Rectal-Use of suppositories is reserved for situations in which oral administration is difficult (Patient not conscious, difficulty swallowing from esophageal disease). Frequently used in small children. 
  2. Parenteral Routes
    1. Intravenous injection- Used when a rapid clinical response is necessary, e.g., an acute asthmatic episode, the intravenous (I.V.) route is the preferred one.
    2. Intra-arterial injection-used used notably with anticancer drugs.
  1. Intrathecal injection- given via lumbar puncture and injection into the subarachnoid space.
  2. Intramuscular injection- Drugs may be injected intramuscular (I.M.) into the arm (deltoid), thigh (vastus lateralis) or buttocks (gluteus maximus). 
  3. Subcutaneous injection- Some drugs, notably insulin and some vaccines, are routinely administered subcutaneously (S.C.) 
  4. Inhalation-  drugs which affect pulmonary function, are administered as aerosols. For example,  inhalers for asthma.

-When a user snorts a drug, they inhale the substance — usually as a powder or crushed pill — through the nose. This mode of administration is also referred to as nasal insufflation.

-Sniffing — not to be confused with snorting — is the term associated with inhalant abuse. Inhalants can be gases, solvents, aerosols, and nitrites.  

  1. Topical application – usually topical administration means application of the medication to body surfaces such as directly to the skin (epicutaneous ) or mucous membranes.  Topical formulations include creams, foams, gels, lotions, and ointments. Topical medication are applied  
    1. Eye -for local effects.
    2. Ear-for conditions of the middle or inner ear
    3. Intravaginal- for infections or contraceptives.
    4. Intranasal -for alleviation of  nasal congestion
  2. Skin-Topical drug administration for skin disorders, for example eczema

Pharmacokinetics and pharmacodynamics

Pharmacokinetics: refers to all the aspects relating to “What the body does to a drug, ” including drug absorption, distribution, metabolism, and excretion/ elimination.

  • Absorption: The drug absorption from the site of administration which permits the entry of the therapeutic agent into the plasma. Absorption is the transfer of a drug from the site of administration to the bloodstream via one of several mechanisms. Absorption rate of a drug depend on, the environment where the drug is absorbed, chemical characteristics of the drug (e.g., water soluble, fat soluble), and the route of administration. For an Intravenous drug,  no absorption occurs . Distribution occurs immediately after administration
  • Distribution: reversible process, the drug leaves the bloodstream and distributes into the interstitial and intracellular fluids.
  • Bioavailability: the fraction of administered drug that reaches the systemic
  • Metabolism: biotransformation of the drug into metabolites by the liver or other tissues
  • Elimination: Once the drug enters the body, elimination begins, the drug and its metabolites are excreted or eliminated into urine, bile or feces. Metabolism leads to products with increased polarity which allows drug elimination.  Elimination also depends on the amount of the drug delivered to the liver or the kidney per time unit. The most important route for drug removal from the body is through the kidney into the urine.  

When an oral drug is absorbed across the GI tract, it first enters the portal circulation before the systemic circulation.  If the drug is rapidly metabolized, less of the active ingredient will reach the systemic circulation. This is called first pass effect, a phenomenon of drug metabolism whereby the concentration of a drug, specifically when administered orally, is greatly reduced before it reaches the systemic circulation. Example: nitroglycerine, (90% is cleared through passage through the liver). Nitroglycerin sublingual tablets are used to treat episodes of angina pectoris (chest pain) in people who have coronary artery disease (narrowing of the blood vessels that supply blood to the heart). 

 Pharmacodynamics: describes what the drug does to the body

Pharmacodynamics- covers all the aspects relating to “What a drug does to the body”.  It is the study of the mechanism by which the drug produces a response, including action: how and where the effect is produced ; effect: the type of response (stimulation, inhibition) produced by drug.  Drugs target receptors because receptors are involved in chemical signaling between and within cells. Receptors can be located on the cell membrane, within the cytoplasm of the cell, or on cell nuclei.   The molecules (drugs, supplements, chemicals, hormones, neurotransmitters, peptides) that bind to receptors are called ligands. Ligands bind to specific regions of receptors, called recognition sites. The binding site for an exogenous (not present in your body) drugs may be the same as or different from endogenous (present in your body) ligands (hormone or neurotransmitter). A ligand that activates a receptor is called an agonist. The activation of receptors can increase or decrease (inhibit) particular cell functions that are already ongoing or the ligand can start a cell process from the beginning. A ligand that blocks a receptor is called an antagonist. 2x

Dose Response Curve (Figure 1)  - When a drug is administered, it produces a response and this response shows alteration with change in dose. The change can be plotted on graph known as Dose Response Curve.  The dose response curves differ with administering an analgesic or depressant dose of morphine.

Potency- It is used to indicate the amount of drug required to produce a specific response. Potency of a drug is different from it strength. The strength is the amount of drug in the dosage form or a unit of the dosage form (e.g. 500 mg capsule, 250 mg/5 mL suspension).

Efficacy- is the maximal effect produced by a drug .  Synergy-Action of one drug is increased by the other. 

Figure 1

Figure 1. Dose–response curves for an analgesic and depressant doses of morphine. 

Chapter Review and Discussion Questions

    1. Describe the common routes of drug administration
    2. What is pharmacokinetics and how does it differ from pharmacodynamics.
    3. Define efficacy, potency, and strength of a drug.
    4. Define agonists and antagonists and its application in treating a drug overdose.
    5. What is a first pass-effect ?  Why is a first pass effect important?