This is a
very good question.
Because this can be somewhat technical, and hinges on a couple of variables, it is an often misunderstood area so the two terms (half-life & active-life) are generally and erroneously used as synonyms.
The half-life defines the time required for elimination of half the amount of substance absorbed, i.e. for plasma levels to drop by 50%. However, the action of a drug ceases when it disappears from its action site, as in your experience with Clen's side effects. This can occur by two mechanisms:
- By redistribution, the mechanism by which the drug is initially distributed to the action site, but is also subsequently distributed to peripheral storage areas, mainly fat tissue, skeletal muscle and liver.
- By drug elimination, including both excretion and metabolic inactivation of the active ingredient.
Here's where it gets a semi-complicated, "Clearance" quantifies the capacity of the body to eliminate the drug, and can be defined as the volume of blood or plasma that is cleared of a substance through elimination processes.
Thus, the half-life can be regarded as the result of two primary processes:
- The distribution capacity of the drug, expressed by its distribution volume.
- The elimination capacity of the body, expressed by clearance, so that the elimination half-life increases with increasing distribution volumes and decreases with a decreasing clearance.
Hence the old adages, "Because you can't feel it doesn't mean it's not working/there", and the doctors often quoted antibiotic creed, "Continue to take this even when your symptoms have dissipated."