Pharmacodynamics deals with the drug’s action and effect within the body. After administration, most drugs enter the systemic circulation and expose almost all body tissues to possible effects of the drug.All drugs produce more than one effect in the body. The primary effect of a drug is the desired or therapeutic effect. Secondary effects are all other effects, whether desirable or undesirable, produced by the drug. Most drugs have an affinity for certain organs or tissues and exert their greatest action at the cellular level on those specific areas, which are called target sites.
There are two main mechanisms of action:
1. Alteration in cellular environment
2. Alteration in cellular function

Alteration in Cellular Environment
Some drugs act on the body by changing the cellular environment, either physically or chemically. Physical changes in the cellular environment include changes in osmotic pressures, lubrication, absorption, or the conditions on the surface of the cell membrane. An example of a drug that changes osmotic pressure is mannitol, which produces a change in the osmotic pressure inbrain cells, causing a reduction in cerebral edema. A drug that acts by altering the cellular environment by
lubrication is sunscreen. An example of a drug that acts by altering absorption is activated charcoal, which is administered orally to absorb a toxic chemical ingested into the gastrointestinal tract. The stool softener docusate is an example of a drug that acts by altering the surface of the cellular membrane. Docusate has emulsifying and lubricating activity that causes a lowering of the surface tension in the cells of the bowel, permitting water and fats to enter the stool. This softens the fecal mass, allowing easier passage of the stool. Chemical changes in the cellular environment include inactivation of cellular functions or the alteration of the chemical components of body fluid, such as a change in the pH. For example, antacids neutralize gastric acidity in patients with peptic ulcers.
Alteration in Cellular Function
Most drugs act on the body by altering cellular function. A drug cannot completely change the function of a cell, but it can alter its function. A drug that alters cellular function can increase or decrease certain physiologic functions, such as increase heart rate, decrease blood pressure, or increase urine output.

Receptor mediated drug response

A cell alters its function under the influence of a drug. a receptor is a macromolecule which is present on the surface of the cell which is the reason for interaction of the drug with the cell. this yields a response which is the therapeutic response of the drug. The intensity of a drug response is related to how good the “fit” of the drug molecule is and the number of receptor sites occupied.

Antagonists

Antagonists prevent receptor activation. Preventing activation has many effects. Antagonists increase cellular function if they block the action of a substance that normally decreases cellular function. Antagonists decrease cellular function if they block the action of a substance that normally increases cellular function.

Receptor antagonists can be classified as reversible or irreversible. Reversible antagonists readily dissociate from their receptor; irreversible antagonists form a stable, permanent or nearly permanent chemical bond with their receptor (eg, by alkylation). Pseudo-irreversible antagonists slowly dissociate from their receptor.

In competitive antagonism, binding of the antagonist to the receptor prevents binding of the agonist to the receptor.

In noncompetitive antagonism, agonist and antagonist can be bound simultaneously, but antagonist binding reduces or prevents the action of the agonist.

In reversible competitive antagonism, agonist and antagonist form short-lasting bonds with the receptor, and a steady state among agonist, antagonist, and receptor is reached. Such antagonism can be overcome by increasing the concentration of the agonist. For example, naloxone (an opioid receptor antagonist that is structurally similar to morphine), when given shortly before or after morphine, blocks morphine’s effects. However, competitive antagonism by naloxone can be overcome by giving more morphine.

Structural analogs of agonist molecules frequently have agonist and antagonist properties; such drugs are called partial (low-efficacy) agonists, or agonist-antagonists. For example, pentazocine activates opioid receptors but blocks their activation by other opioids. Thus, pentazocine provides opioid effects but blunts the effects of another opioid if the opioid is given while pentazocine is still bound. A drug that acts as a partial agonist in one tissue may act as a full agonist in another.

Agonists

Agonist activate receptors to produce the desired response. Conventional agonists increase the proportion of activated receptors. Inverse agonists stabilize the receptor in its inactive conformation and act similarly to competitive antagonists (see Agonists and antagonists). Many hormones, neurotransmitters (eg, acetylcholine, histamine, norepinephrine), and drugs (eg, morphine, phenylephrine, isoproterenol, benzodiazepines, barbiturates) act as agonists.

Receptor-Mediated Drug Effects
The number of available receptor sites influences the effects of a drug. If only a few receptor sites are occupied, although many sites are available, the response will be small. If the drug dose is increased, more receptor sites are used and the response increases. If only a few receptor sites are available, the response does not increase if more of the drug is administered. However, not all receptors on a cell need to be occupied for a drug to be effective. Some extremely potent drugs are effective even when the drug occupies few receptor sites.