My five years old started with a cold sore for the first time. Is there any medication or treatment for this condition?

Oral herpes? First confirm the diagnosis. Not all apparent cold sores are due to herpes. A sore inside the mouth more likely is a canker sore, not a cold sore. Effective treatment is available for oral herpes, but not before confirming the diagnosis and it needs a prescription. Take your daughter to the pediatrician right away -- the sooner the better for accurate diagnosis and for effective treatment.