Paul,
 Pharmokinetics can vary, if the drug has active metabolites, if it
is retained in tissues (like the drugs for osteoporosis), but in
general you have it right. The half life has to due with serum levels.
Here's a reference.
http://www.medscape.com/viewarticle/408238_4
Judy

Susan,
the experience you have described is probably not representative of most
patients.

Unless they're extraordinarily sensitive to any level of a medication
(such as in the case of severe antibiotic allergy), after a few
half-lives have passed the effective dose is too small to really matter.

Most likely, what the OP was wondering about was the claim by the
manufacturer of azithromycin that its half-life is so long that he only
needs to take 5 days' worth (the infamous Zithromax "Z-pak").  With a
half-life of 65 hours, that would be roughly the same as taking some
other antibiotic for 6 or 7 days.  That's still nowhere long enough to
knock out a stubborn sinus infection.

In general, I'm not a fan of standard dose-paks, such as those sold for
Zithromax and Medrol.  The physician should prescribe the medication at
the dose and duration needed by the specific patient for the specific
condition.

Thanks to all for replying , I understand it better now
Cheers Paul