Sustained vs. immediate release
In order to avoid high peak plasma drug levels after repeated oral administration, sustained release tablets were developed. Besides the avoidance of toxic high peak concentrations, sustained release tablets also prolong the duration of drug action.
Examples of drugs available as sustained release tablets (or retard tablets) are theophylline, nifedipine, and verapamil.
See how different dosage forms and regimens will effect serum concentrations by clicking here