Inclusion Criteria:
* Men and women aged 18-80 years who are able and willing to provide written informed consent and to comply with the requirements for the entire study
* Satisfy Rome III criteria for functional constipation and/or irritable bowel syndrome as assessed by questionnaires and objective evidence of dyssynergic defecation on anorectal testing
Exclusion Criteria:
* Clinical evidence of significant cardiovascular, respiratory, renal, hepatic, gastrointestinal, hematological, neurological, psychiatric or other diseases that may interfere with the objectives of the study and/or pose safety concerns.
* History of total, hemi- or subtotal colectomy at any time or other abdominal operations within 60 days prior to entry into the study. Segmental resection of the colon (e.g. sigmoid resection) for diverticulitis will be permitted.
* Current anal fissure
* Clinically significant structural abnormalities (eg, rectocele \> 4cm, large enterocele, full thickness rectal prolapse) that, in the opinion of the investigator, are likely contributing to symptoms
* Current or past history of colon or rectal cancer, scleroderma, inflammatory bowel disease, congenital anorectal abnormalities, solitary rectal ulcer syndrome, rectal resection or pelvic irradiation
* Pregnant or nursing women, prisoners and institutionalized individuals
* BMI \>35 kg/m2
* Use of opioid analgesics or GLP-1 receptor agonists. Patients on opioids will be allowed to participate if they have discontinued them for at least 1 week before the date of screening and are willing to stay off them for the duration of the study.
* Current use of anticholinergics (eg. amitriptyline, hyoscyamine). Patients who use low dose tricyclic antidepressants (nortriptyline or amitriptyline up to 50 mg/day) will be eligible provided they do not increase the dose during the study period. Patients on higher doses are eligible to participate if it is safe and they are willing to reduce their medication dose at least 3 days (72 hrs) before the date of screening and are willing to stay off them till the study is complete.
* Current use of laxatives (Ex-lax, senna, herbal supplements), linaclotide, lubiprostone, and prucalopride to treat constipation- patients using these medications will be eligible to participate if they can discontinue the medication at least 3 days (72 hrs.) before the date of screening and are willing to stay off them till the study is complete (As detailed later, all participants can use Miralax throughout the study).
* Any other factors that increase the risk to participants of participating in the study and/or undermine the integrity of the data.
