Metronidazole has a half life of eight hours in healthy adults, explains Drugs.com. Drugs with a half life of eight hours reduce to a statistically insignificant amount after 40 hours, though it takes up to 80 hours for the drug to be completely eliminated.
Drugs.com states that up to 80 percent of a metronidazole dosage is eliminated through urine, while about 15 percent is eliminated through excretion.
Newborn infants have a reduced ability to eliminate metronidazole. The drug can have a half life of more than 100 hours in infants with a gestational age of 28 to 40 weeks. Elderly people, especially those with liver problems, are more likely to experience a slower half life and adverse effects from metronidazole, explains Drugs.com.