This is a common scenario in pediatric wards: A child is admitted for pneumonia and receives antibiotic treatment for several days. His condition improves. After about 3 to 4 days, he is discharged from the hospital and the mother is instructed to continue giving oral antibiotic to his son until a 7-day course of antibiotic treatment is completed. Several days after discharge, the mother is compelled to have his son readmitted for another reason. This time the child is having diarrhea.
Most of us would think that another bout of infection might have caused the child’s acute onset of diarrhea. But be evenhanded, this impression is not always true. Not another round of antibiotic can treat the diarrhea. Antibiotics can even worsen it.
The human gut is normally inhabited by several species of nonpathologic (nondisease-causing) bacteria which affords many health benefits. The existence of these good bacteria (about 400 types) in the lining of intestines has protective functions and promotes a healthy digestive system. It reduces the growth and over proliferation of harmful disease-causing bacteria such as E.coli, Yersinia, and Helicobacter pylori in the intestines by increasing luminal acidity, secreting bactericidal protein, and inhibiting bacterial adhesion to the intestinal wall.
In most cases, when an antibiotic is given to kill the harmful bacteria that cause a certain illness like pneumonia, the good bacteria are also eradicated. Its population in the intestines is significantly reduced. This will then allow growth and propagation of harmful bacteria, most commonly Clostridium difficile, which causes diarrhea. This type of diarrhea is commonly referred to as antibiotic-induced diarrhea.
Nowadays, with the advent of scientific technology, these good bacteria are cultured and made available in the market. They are collectively called as “probiotics”. Probiotics are especially prepared products containing viable, defined microorganisms in sufficient numbers. They come in the form of capsules, tablets, liquid, powder, or sometimes incorporated into food products.
The most commonly known food products with live microorganisms are therapeutic yogurt preparations. Yogurts are more appetizing, hence well-tolerated by most patients, especially young children. Yogurts contain Lactobacillus acidophilus which is the largest population of probiotic bacteria in the human intestine. 
Clinical studies have established dosages with therapeutic efficacy. For children, the frequently used dosage range from 5 to 10 billion colony-forming units per day. For adults, dosage range from 10 to 20 billion colony-forming units per day. These figures will vary though, based on the specific type of microorganisms used.  Species of microorganisms with widespread use and with most clinical testing are Lactobacillus species, Bifidobacterium species, and Saccharomyces boulardii.
Based on a recent scientific research data, use of probiotics reduces the risk of antibiotic-induced diarrhea by 52% and that the best time to administer it is within 72 hours of starting antibiotic treatment. 
In summary, giving probiotic supplements like yogurt to your child may help replace the loss of beneficial bacteria, and thus help decrease the severity and duration of diarrhea. Even without diarrhea, probiotics like yogurt can safely be given to children. They neither cause allergic reactions nor cross react with other medications. In fact, they are also proven effective in preventing occurrence of infectious diarrhea caused by viruses and harmful opportunistic bacteria.
1. Monica Boirivant; Warren Strober (2007). The Mechanism of Action of Probiotics.
2. Laurie Barclay, MD (2008). Benefits of Probiotics Reviewed.