Long-term Safety of Linaclotide in Pediatric Participants With FC or IBS-C

Inclusion Criteria: * Participant weighs ≥ 18 kg at the time the parent/guardian/LAR and/or caregiver has provided signed consent. * Female participants who have had their first menstrual period and are sexually active must agree to use a reliable form of contraception. * Participants must have completed study intervention in their lead-in study. Inclusion Criteria for Phase 2 LIN-MD-62 or Phase 2 LIN-MD-63 and Phase 3 LIN-MD-64 completers who enroll in LIN-MD-66 after \>28 days from last study intervention: \- Female participants of childbearing potential must have a negative serum pregnancy test at the Screening Visit (Visit 1) and negative urine pregnancy test prior to the first dose on the Day 1 Visit (Visit 2). Exclusion Criteria: * Participant has a known allergy or sensitivity to the study intervention or its components or other medications in the same drug class. * Participant received an investigational drug, other than linaclotide, during the 30 days before the Screening Visit (Visit 1) or is planning to receive an investigational drug (other than that administered during this study) or use an investigational device at any time during the study. * Female participants who are currently pregnant or nursing, or plan to become pregnant or nurse during the clinical study. * Participant has required manual disimpaction any time prior to study intervention or disimpaction during in-patient hospitalization within 1 year prior to study intervention. * Participant has any of the following conditions: * a) Down's syndrome or any other chromosomal disorder * b) Anatomic malformations (eg, imperforate anus, anal stenosis, anterior displaced anus) * c) Intestinal nerve or muscle disorders (eg, Hirschprung disease, visceral myopathies, visceral neuropathies) * d) Neuropathic conditions (eg, spinal cord abnormalities, neurofibromatosis, tethered cord, spinal cord trauma) * e) Neurodevelopmental disabilities (early-onset, chronic disorders that share the essential feature of a predominant disturbance in the acquisition of cognitive, motor, language, or social skills, which has a significant and continuing impact on the developmental progress of an individual) producing a cognitive delay that precludes comprehension by the participant. * Participant has a mechanical bowel obstruction or pseudo-obstruction. * Participant currently has both unexplained and clinically significant alarm symptoms (lower GI bleeding \[rectal bleeding or heme-positive stool\], iron-deficiency anemia, or any unexplained anemia, or weight loss) and systemic signs of infection or colitis, or any neoplastic process. * Participant has an active anal fissure (Note: history of anal fissure is not an exclusion). * Participant has had surgery that meets any of the following criteria: * a) Bariatric surgery for treatment of obesity, or surgery to remove a segment of the GI tract at any time before the Screening Visit (Visit 1). * b) Surgery of the abdomen, pelvis, or retroperitoneal structures during the 6 months before the Screening Visit (Visit 1) * c) An appendectomy or cholecystectomy during the 60 days before the Screening Visit * d) Other major surgery during the 30 days before the Screening Visit (Visit 1) * Participant is receiving enteral tube feeding * Participants who have positive urine drug screen results for cocaine, barbiturates, opiates, or cannabinoids will be excluded from study participation. Exclusion Criteria for LIN-MD-62, LIN-MD-63 and LIN-MD-64 Completers Who Enroll in LIN-MD-66 \> 28 Days From Last Study Intervention: * Participant has a history of nonretentive fecal incontinence * Participant has a history of drug or alcohol abuse * Participant has any of the following conditions: * a) Celiac disease, or positive serological test for celiac disease and the condition has not been ruled out by endoscopic biopsy * b) Cystic fibrosis * c) Hypothyroidism that is untreated or treated with thyroid hormone at a dose that has not been stable for at least 3 months prior to the Screening (Visit 1) * d) Lead toxicity, hypercalcemia * e) Inflammatory bowel disease * f) Childhood functional abdominal pain syndrome * g) Childhood functional abdominal pain * h) Poorly treated or poorly controlled psychiatric disorders that might influence his or her ability to participate in the study * i) Lactose intolerance that is associated with abdominal pain or discomfort and could confound the assessments in this study * j) History of cancer other than treated basal cell carcinoma of the skin. (Note: Participants with a history of cancer are allowed provided that the malignancy has been in a complete remission for at least 5 years before the Randomization Visit. A complete remission is defined as the disappearance of all signs of cancer in response to treatment.) * k) History of diabetic neuropathy * Participants who have positive urine drug screen results for cocaine, barbiturates, opiates, or cannabinoids.