Most penicillin and cephalosporin meds are taken at 500mg dosages 3-4 times a day for adults, (250mg dosages for children) and will treat many skin and respiratory infections. Metronidazole (250mg) and Doxycycline (100mg) are taken twice a day and will treat many types of diarrheal disease. Fish Mox comes in a pill dosage of 250mg and Fish Mox Forte comes in 500mg. According to Dr. Bones and Nurse Amy, an Amoxicillin dose is usually administered 3 times a day. Usually, 20-50 mg per kilogram of weight (20-30 mg per kilogram for children less than 4 months old).