The goal of this clinical trial is to learn if plecanatide can treat functional constipation in children and adolescents aged 6 to less than 18 years. It will also learn about the safety and pharmacokinetics of plecanatide in this population. The main questions it aims to answer are:
* Does plecanatide increase the number of spontaneous bowel movements compared to placebo after 8 weeks of treatment?
* What medical problems do participants experience when taking plecanatide? Researchers will compare low-dose plecanatide and high-dose plecanatide to placebo to see if plecanatide improves bowel movement frequency, stool consistency, and constipation-related symptoms.
Participants will:
* Complete a screening period with daily electronic diary entries to record bowel movements, symptoms, and rescue medication use
* Take plecanatide or placebo by mouth once daily for 8 weeks
* Continue daily electronic diary entries throughout the study
* Attend clinic visits for physical exams, laboratory tests, and assessments of symptoms and safety
* Provide blood samples for pharmacokinetic and safety evaluations
* Complete questionnaires about constipation symptoms and quality of life