Drugs.com explains, Omnicef, also known as cefdinir, has a half-life of 1.7 to 1.8 hours with normal kidney function. This means that in 1.7 or 1.8 hours half of the dosage taken is removed from one's system. In an additional 1.7 to 1.8 hours only one-quarter of the dose remains.
This cephalosporin antibiotic is excreted primarily by the kidneys. If the kidneys are impaired, they are less able to clear the Omnicef. According to the manufacturer, Teva Pharmaceuticals, it could take up to 5 times longer than normal for it to clear the body entirely.
The American Society of Health-System Pharmacists notes that the absorption of Omnicef is decreased when it is taken with antacids or iron supplements. Therefore, the kidneys are required to clear a reduced amount of the drug.