This prospective, randomized, parallel-group, double blind, single-center study is to be conducted in the Medical University of Warsaw Public Paediatric Teaching Hospital in following departments: Department of Paediatric Gastroenterology and Nutrition, Department of Paediatrics and Nephrology, Department of Pediatric Pneumonology and Allergy, Department of Pediatrics with Medical Assessment Unit, Admissions Department. It is planned to include a total of 156 children aged between 12 months to 18 years old receiving antibiotic therapy because of acute respiratory tract infection/or urinary tract infection. The children will be randomly assigned to receive 100 mg of bovine lactoferrin or placebo twice a day orally for the whole period of antibiotic therapy. The primary efficacy parameter is occurrence of diarrhea during the antibiotic therapy and two weeks after, defined as > 3 stools a day, a watery or loose stool with/or occurrence of blood in the stool.

Inclusion Criteria:

• acute respiratory or urinary infection requiring empiric antibiotic therapy.
• antibiotic treatment started before 24 hours prior to enrollment.
• the consent of parents or legal guardians to participate in the study

Exclusion Criteria:

• severe or generalized bacterial infection
• antibiotic therapy during the last 8 weeks
• use of probiotic during the seven days prior to enrollment
• immune disorders,
• chronic disease of the gastrointestinal tract,
• actual acute or chronic diarrhea,
• intake of iron supplementation,
• cows milk protein allergy
• lack of consent of the parents or legal guardians to participate in the study.