Dosage Regimen: Fixed Dose

Most drugs are administered in multiple fixed doses repeated at defined intervals to achieve and maintain safe and therapeutic levels of the drug in the body.

Such fixed-dose regimens can be followed for both intravenous and oral medications. For example, one unit of a drug can be injected twice daily, or a pill can be taken three times a day.

When the first dose of a drug is administered, its plasma concentration peaks and then starts declining due to elimination. As the concentration falls to 50% of the starting value, the next dose is administered to raise the drug's plasma levels again. 

Such a repetitive dosage cycle allows the body to accumulate the drug until a steady state is achieved, where the rate of administration becomes the same as the elimination rate.

A well-designed fixed dosage regimen avoids excessive fluctuations of drug concentrations, remaining within a safe therapeutic range to generate an effective and lasting response.