Evaluation of Clinical Equivalence Between Two Lubiprostone Products

Main Inclusion Criteria: 1. 1\. Male or non-pregnant females aged 18 and older with a clinical diagnosis of chronic idiopathic constipation defined as, on average, \< 3 spontaneous bowel movements (SBMs) per week for the past 6 months and confirmed by daily diary during the 14 day baseline lead-in period. An SBM is defined as any bowel movement that did not occur within 24 hours after rescue medication use. 2. Have 1 or more of the following symptoms related to bowel movements for at least 6 months before the baseline visit and confirmed by daily diary during the 14 Day baseline period: * very hard (little balls) and/or hard stools for at least 25 percent of the bowel movements * sensation of incomplete evacuation following at least 25 percent of the bowel movements * straining at defecation at least a quarter of the time 3. Women of child-bearing potential must have a negative pregnancy test prior to beginning therapy and agree to use effective contraceptive methods during the study. 4. For patients aged \< 50 years, documentation of the results of either a flexible sigmoidoscopy or colonoscopy performed within the 5 years prior to dosing, showing no mechanical bowel obstruction or organic disorders of the large Bowel. 5. For patients aged \>/= 50 years, documentation of the results of either a barium enema with flexible sigmoidoscopy or colonoscopy performed within 1 year prior to dosing, showing no mechanical bowel obstruction or organic disorders of the large bowel. Main Exclusion Criteria: 1. Females who are Pregnant, breast feeding, or planning a pregnancy. 2. Patients of any age with evidence of weight loss, anemia, or rectal bleeding AND without documentation of the results of either a flexible sigmoidoscopy or colonoscopy performed during the 6 months prior to dosing. 3. Patients that have documented mechanical bowel obstruction (e.g., bowel obstruction due to tumor, hernia), megacolon/megarectum, or diagnosis of pseudo-obstruction. 4. Any known or suspected organic disorders of the large or small bowel (e.g., inflammatory bowel disease, ulcerative colitis, Crohn's Disease) or constipation secondary to a documented cause (e.g., surgery, bowel resection)or acute hernia. 5. History of bowel resection. 6. Patients who are regularly using medications know to cause constipation (narcotics, calcium channel blockers, tricyclic antidepressants). 7. Hospitalized for any gastrointestinal or abdominal surgical procedure during the 3 months prior to dosing. 8. Clinically significant cardiovascular, liver, lung, neurologic, renal or psychiatric disorder, or clinically significant laboratory abnormalities. 9. Use of a systemic antibiotic within 4 weeks prior to the Screening Visit. 10. Participation in a study with any investigational medication within the past 30 days of screening for this study or previous participation in this study.