Minor side effects are appetite loss, drowsiness, diarrhea, metallic taste, headaches, stomach upset and vomiting. Antibiotics in treating abscesses in the liver, pelvis, abdomen and brain caused by susceptible anaerobic bacteria, amebiasis, trichomoniasis and asymptomatic partners of infected patients. Antibiotics will not work for colds, flu, or other viral infections. Metronidazole selectively blocks some of the functions within the bacterial cells and the parasites resulting in their death. It is important to maintain a constant level of Flagyl in the blood. Metronidazole should be used with caution in patients with active or chronic severe peripheral and central nervous system diseases due to the risk of neurological aggravation. Metronidazole should not be used in the first trimester of pregnancy. Metronidazole is usually taken two or three times a day for ten days or longer.