Intravenous Administration (IV)

Intravenous administration places the entire administered amount into the systemic circulation with no absorption step, which is why its bioavailability is defined as one hundred percent and every other route is expressed as a fraction of it. Two patterns are used. A bolus produces the highest achievable peak almost immediately and is the classical way to measure volume of distribution and clearance. An infusion delivers at a controlled rate, so concentration climbs toward a plateau set by the ratio of rate to clearance, arriving after several half-lives.

For peptides the route is more often a research and hospital tool than a therapeutic default. Physiological studies of the incretin effect depend on intravenous infusion of glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide, which is how their very short native half-lives were characterised. Clinically, octreotide is given intravenously in acute settings, and vasopressin and its analogues are infused in critical care.

Removing absorption as a variable makes the route both precise and unforgiving. There is no depot to buffer an error and no absorption phase to blunt a peak, so the full consequence of a rate or concentration error arrives at once. Anything entering a vein must also meet a higher standard than other routes: sterile, essentially free of visible particulates, and near physiological tonicity and pH.

Two errors recur. The first is treating intravenous data as transferable to other routes; a subcutaneous profile has a different peak, a different time to peak and often a different apparent half-life, so intravenous figures quoted for a subcutaneously administered peptide mislead. The second is neglecting the hardware, where adsorption to tubing and filters removes a meaningful fraction of a dilute peptide before it reaches the patient.