• Agonist
An agonist is a compound that binds to a receptor and produces the biological response.

• Efficacy
Efficacy is the maximal response a drug can produce. Potency is a measure of the dose that is required to produce a response.

• Potency
Potency is often expressed as the dose of a drug required to achieve 50% of the desired therapeutic effect. This is the ED50 (effective dose).

A partial agonist produces the biological response but cannot produce 100% of the biological response even at very high doses.

• Therapeutic Index
Therapeutic index is a measure of drug safety. A drug with a higher therapeutic index is safer than one with a lower therapeutic index.

• Antagonist
Antagonists block or reverse the effect of agonists. They have no effect of their own.

Competitive antagonists make the agonist look less potent by shifting the dose response curve to the right.

A noncompetitive antagonist reduces the maximal response that an agonist can produce.

Inverse agonists have opposite effects from those of full agonists. They are not the same as antagonists, which block the effects of both agonists and inverse agonists.

• First Pass Effect

The liver is a metabolic machine and often inactivates drugs on their way from the GI tract to the body. This is called the first-pass effect.

Drugs cross membranes by passive diffusion or active transport.

A drug tends to pass through membranes if it is uncharged.

For a weak acid, when the pH is less than the pK, the protonated form (nonionized) predominates. When the pH is greater than the pK, the unprotonated (ionized) form predominates.

For a weak base, when the pH is less than the pK, the ionized form (protonated) predominates. When the pH is greater than the pK, the unprotonated (nonionized) form predominates.

In the stomach (pH 2.0), weak acids are uncharged and will be absorbed into the blood- stream, whereas weak bases are charged and will remain in the GI tract.

Bioavailability is the amount of drug that is absorbed after administration by route X compared with the amount of drug that is absorbed after intravenous (IV) administra- tion. X is any route of drug administration other than IV.