Participants with IBS are skin patch tested (no needles) to 80 different foods and food additives, compounded for patch testing, in search of food allergies. The testing requires 3 office visits within a 4 or 5 day period. The patches are taped to the back at Visit #. At Visit #2 48 hours later, the patches are removed from the skin and the outside border of each patch is marked with a felt tip marker. At Visit #3 (final visit) 1 or 2 days later, the patch test reading is performed by the doctor-investigator. An allergy is identified as a small red mark where the food was in contact with the skin for 48 hours. Those participants found to have food allergies are then placed on an avoidance diet (no calorie restriction) for 16 weeks where they either avoid eating the food(s) to which they are allergic (the "true" avoidance diet) or food(s) to which the testing did not show an allergy (this is called the "sham" avoidance diet). There is a 50/50 chance of going on either avoidance diet. The avoidance diet is assigned in such a way that neither the participant or the doctor-investigator knows which diet is being followed. After the 16 weeks, the participants answers a brief online questionnaire that asks about the IBS symptoms while following the avoidance diet. After the 16 week avoidance diet and final questionnaire are completed, those participants who were on the sham diet will be told of their true food allergies which they may try avoiding on their own.

See above. When all 250 study participants have finished all parts of the study, the results will be made known to the doctor-investigators for data analysis.

Inclusion Criteria:

1. Irritable bowel syndrome diagnosed by a primary care provider, gastroenterologist or allergist; or
2. meeting the Rome IV IBS diagnostic criteria by history and having suboptimally or poorly controlled IBS symptoms.

Exclusion Criteria:

1. Under age 18 years
2. Pregnant
3. Severe rash
4. Receiving any cortisone-containing or any of the following immunosuppressive medications within two weeks prior to patch testing or plan to do so any time during the study: cyclosporine, mycophenylate mofetil, azathioprine, tacrolimus or others within these classes of medications)
5. Incapable of completing all parts of the 18-week screening period and study, including following the dietary instructions and completing all text or email questionnaires
6. Exposure of back to the sun in the 2 weeks prior to patch testing Unable or unwilling to discontinue the low FODMAP diet, if relevant, starting 1 week prior to the study and for the study duration

8) Refusal to shave back hair, if relevant 9) Receiving pharmacologic therapy for IBS that has been started or changed within 30 days of study enrollment 10) Non-English speaking 11) Unable to provide written informed consent 12) Have a history of gastrointestinal disease including celiac disease, cirrhosis, gastrointestinal malignancy, inflammatory bowel disease, or diverticulitis, active within the prior 2 years. 13) Have a history of gastrointestinal surgery (except appendectomy and cholecystectomy or gallbladder removal >6 months ago) 14) Have poorly controlled psychiatric disease such as severe depression (with or without suicidal ideation), severe anxiety, schizophrenia, dementia 15) Have excessive alcohol intake (more than 1 drink per day for females and 2 drinks per day for males) 16) Use illicit substances 17) Use high-dose opiates 18) Severe allergy to adhesive tape

The true or sham avoidance diet for each participant is randomly assigned by the study administrator. Assignments will not be made available to participants until after their study completion or to investigators until all 250 participants have completed the study and the results have been analyzed.