Drug+Action+&+Advers+Reactions

Drugs can be classified based on the following properties: biochemical action, physiological effects, or organ systems involved.
=== Potency of a drug is related to the amount of the drug required to produce an effect. Efficacy is related to the maximal effect of a drug; regardless of the dose. Once the maximal effect of the drug is reached administering more of the drug will increase the posibility of toxicity. ===

Every drug has more than one action: An adverse drug reactions is a response to a drug that is not desired, is potentially harmful, and occurs at usual therapeutic doses. Adverse drug reactions can be divided into the following categories: It is important to distinguish between the different adverse reaction types because they are significant and become pertinent when discussing an adverse reaction with another health professional. It is important to describe the patient's "problem" to the drug in enough detail so that the adverse reactions can be separated from the allergic reactions.
 * 1) the clinically desirable actions which is called therapeutic dose
 * 2) the undesirable reactions are called adverse reactions
 * Toxic reaction- is an extension of the pharmacologic effect resulting from a drug's effect on the target organs. The amount of the desired effect is excessive.
 * Side effect- a dose-related reaction that is not part of the desired therapeutic outcome. Occurs when the drug acts on non-target organs to produce undesirable effects. Side effect and adverse reaction are commonly used interchangeably. The drowsiness produced when an antihistamine is taken is an adverse reaction.
 * Idiosyncratic reaction- a genetically related abnormal drug response. Certain populations because of their genetic constitution are more susceptible to certain adverse reactions to specific drugs. For example, Eskimos metabolize certain drugs faster than other populations therefore a larger dose of those drugs would be needed in that population.
 * Drug allergy- this is an immunologic response to a drug resulting in a reaction such as a rash or anaphylaxis. This response accounts for less than 5 % of al adverse reactions. These are not predictable and are not dose related.
 * Interfere with natural defense mechanisms- some drugs such as adrenocorticosteriods can reduce the body's ability to fight infection. Drug's that interfere with the body's defenses cause a patient to get infections more easily and have more trouble fighting them off.

Side effects Toxic reactions Allergic reactions Ways that drugs pass through the membrane:
 * Are predictable
 * Dose-related
 * Acts on non-target organs
 * Are predictable
 * Acts on target organs
 * Extensions of pharmacologic effects
 * Non predictable
 * Not dose related

Drugs move across the membrane from a high concentration to low concentration. How fast these drugs cross the membrane is directly dependent on the concentration gradient as well as the lipid solubility of the molecule. The molecule that is more lipid soluble will cross membrane easier and faster. Two methods of specialized transport include: active transport and facilitated diffusion. Active transport is a process by which a substance is transported against a concentration gradient of electrochemical gradient. This action is blocked by metabolic inhibitors. Facilitated diffusion does not move against a concentration gradient. This phenomenon involves the transport of some substances, such as glucose, into cells. It is also blocked by metabolic inhibitors. Pinocytosis may explain the passage of macromolecular substances into the cells.
 * Passive diffusion
 * Specialized transport

Affinity The specificity of drugs cannot be talked about without mentioning the **affinity** of the drugs. The affinity is a measure of how tightly a drug binds to the receptor. If the drug does not bind well, then the action of the drug will be shorter and the chance of binding will also be less. This can be measured numerically by using the dissociation constant KD. The value of KD is the same as the concentration of drug when 50% of receptors are occupied. The equation can be expressed as KD = But the value of KD is also affected by the conformation, bonding and size of the drug and the receptor. The higher the KD the lower the affinity of the drug.

For most drugs used in therapeutic doses the body does not have to use all its capacity to metabolise or eliminate the drug. Instead, a **fixed proportion ** of what ever concentration is presented to the metabolic process is removed. In this situation the concentration declines exponentially with time. (Figure 1). This is termed **1st order kinetics **.
 * First order and Zero order kinetics **

This is not the case for a few therapeutic drugs (phenytoin and alcohol) where all the enzyme capacity is being used ( **saturated **). In this situation a **fixed amount ** of drug is removed and the concentration falls in a linear fashion. ( **Zero order kinetics **) Neutrotransmitters

Neurotransmitters transmit the message across the synapse. The neurotransmitter then interacts with the receptor where they fit together like a lock and key which then the may signal an enzyme to be synthesized or activated then causing a response.