Definitions (according to the Merck Manual)
A drug product is the actual dosage form of a drug, consisting of the drug
itself plus other ingredients formulated into a usable medicine; a tablet,
capsule, or solution.
Drug products must be approved by the Food and Drug Administration before
they can be marketed in the United States. Official drugs standards are then
defined and described in monographs by the United States Pharmacopeia and
legally enforced by the FDA.
Half-life is a very convenient term in biology. It denotes the time required
for the plasma drug concentration or the amount in the body to decrease by
50%. For most drugs, the half-life remains constant regardless of much drug
is in the body. It is related to the elimination constant by
0.693
Half-life = -----------------------------------
Elimination rate constant
Bioavailability is the rate at which and the extent to which the active
moiety (drug or metabolite) enters the general circulation, thereby
permitting access to the site of action.
Absorption is the process of drug movement from the site of application
toward the systemic circulation.
Chemical or pharmaceutical equivalence to drug products that contain the same
compound in the same amount in two or more dosage forms and meet present
official standards.
Bioequivalence refer to chemical equivalents tha, when administered to the
same individual in the same dosage regimen, result in equivalent
concentrations of drugs in blood and tissues.
Therapeutic equivalence refers to two drug products that, when administered
to the same individual in the same dosage regimen, provide essentially the
same therapeutic effect or toxicity; they may or may not be bioequivalent.
I hope this clarifies questions that many subscribers have asked.
Michel Margosis
Former Pharmaceutical Research Chemist
