Irritable bowel syndrome (IBS) is one of the most common gastrointestinal claims among veterans with service-connected PTSD — and one of the most commonly denied. The digestive symptoms are real and well documented, but many claims fail because nothing in the file medically connects the bowel condition to the psychiatric one. That connection is exactly what a nexus letter exists to address.
What "secondary service connection" means here
A secondary claim does not argue that IBS began in service. It argues that a condition the VA has already service-connected — here, PTSD — caused the IBS or aggravated it beyond its natural course. Decades of medical literature on the gut-brain axis describe how chronic stress physiology alters gut motility, visceral sensitivity, and the gut microbiome. IBS is classified as a disorder of gut-brain interaction, and PTSD is among the psychiatric conditions most consistently associated with it. But the VA does not connect those dots on its own. A rating specialist needs a medical opinion in the file that applies that science to this veteran's history.
The four things a strong nexus letter must address
- A current, confirmed IBS diagnosis. Symptoms alone are not enough. The record should show a diagnosis from a treating provider, ideally with other causes of the symptoms considered and reasonably excluded.
- The service-connected primary condition. The PTSD rating decision, its effective date, and the treatment record establishing its severity and course.
- A reasoned medical rationale. The letter must explain the mechanism — how this veteran's PTSD plausibly produced or worsened this veteran's IBS — citing the timeline in the records and the relevant literature, not just asserting a link.
- The correct legal standard. The opinion should state whether the connection is "at least as likely as not" (a 50 percent or greater probability), the threshold VA adjudicators apply.
The evidence problems that sink these claims
Three patterns come up again and again in denials. First, no formal diagnosis — years of documented cramping, alternating bowel habits, and urgency, but no provider ever wrote "IBS" in the chart. Second, a silent timeline: the gastrointestinal complaints predate the PTSD diagnosis in the record, and no one addresses why the aggravation pathway still applies. Third, the conclusory opinion — a one-line statement that the conditions are related, with no mechanism and no discussion of the veteran's actual records. VA raters are instructed to weigh the reasoning of a medical opinion, not just its conclusion. An opinion without reasoning carries little weight, whoever signs it.
Practical steps before you file
Get the diagnosis documented if it is not already. Gather the records that show the sequence: the PTSD diagnosis and rating, the onset or worsening of the bowel symptoms, and any treatment tried. If a physician's stress-related note, a GI workup that ruled out structural disease, or a medication history is buried in your file, flag it — those details often carry the rationale. And be candid about other risk factors; a letter that ignores them is easier for the VA to discount than one that addresses them head-on.
Why the author's credentials matter
Independent medical opinions from Hart Causation and Claims are personally authored by Dr. John H. Hart, Jr., M.D., an ABIM board-certified internist, Lieutenant Colonel, USAF Reserve (Ret.), and former Senior Flight Surgeon. That background combines internal-medicine training — where disorders of gut-brain interaction are diagnosed and managed — with firsthand understanding of military service and the VA's evidentiary standards. The practice writes causation opinions only: no DBQs, no examinations, no diagnosis, and no treatment. Fees range from $1,500 to $3,500 depending on record volume and complexity, standard turnaround is 10–14 business days with a 72-hour rush option, and all communication is HIPAA-compliant through SimplePractice. This article is general information, not medical or legal advice.