What Is a Nexus Letter and Why Do You Need One?

Of the three elements every VA service-connection claim must establish — a current disability, an in-service event, and a link between them — the third one is where most claims are won or lost. The nexus letter is the document that addresses it. Here is what it is, when you need one, who can write one, and what makes the difference between a letter that carries weight and one that doesn't.

What a nexus letter is

A nexus letter is a written medical opinion from a qualified healthcare provider stating whether your current diagnosed condition is connected to your military service (or to another service-connected condition, for secondary claims), with a medical explanation of why.

It is not a treatment record, not a diagnosis, and not a personal statement. It is an opinion document, and its value lies entirely in two things: the qualifications of the person signing it, and the quality of the reasoning inside it.

When a claim needs one

Not every claim requires a nexus letter:

• •Presumptive conditions generally do not — for conditions the law presumes service-connected (Agent Orange, PACT Act burn pit exposures, Gulf War presumptives), qualifying service plus a current diagnosis carries the connection element by statute.
• •Chronic conditions documented in service with continuous treatment since may carry the connection through the records themselves.
• •Most other claims do — especially conditions diagnosed years after separation, and nearly all secondary claims, where the link between two conditions is a medical question that only a medical opinion can answer.

The clearest signal that a claim needs one: a denial letter stating "the evidence does not show a link between your current condition and your military service." That sentence is the VA saying the nexus element is missing.

Who can write one

A nexus opinion must come from a qualified medical professional. That includes:

• •Physicians (MD or DO) — any treating or examining physician
• •Nurse practitioners (NP) and physician assistants (PA)
• •Psychologists (PhD/PsyD) — for mental health conditions
• •Other licensed clinicians opining within their scope of practice

Three points worth knowing:

1. 1.Your own treating provider can write it. There is no rule requiring an "independent" examiner. A treating provider who knows your history often writes the most credible letter — they have the longitudinal record.
2. 2.Specialty relevance matters to weight. A cardiologist's opinion on a heart condition naturally carries more weight than an unrelated specialist's. The opinion should come from someone plausibly qualified on the medical question at issue.
3. 3.The provider signs their own opinion. A nexus letter represents the professional judgment of the clinician who signs it. Drafts and supporting materials can help a provider write efficiently — but the reasoning and conclusion are theirs, and a credible letter reads that way.

What "at least as likely as not" means

VA claims use a unique standard of proof. The phrase that anchors nearly every nexus letter:

"It is my opinion that the veteran's [condition] is at least as likely as not related to [in-service event / service-connected condition]."

Legally, "at least as likely as not" means a 50% or greater probability. This is a deliberately veteran-favorable standard rooted in 38 CFR § 3.102 — the benefit-of-the-doubt rule. When the positive and negative evidence is in approximate balance ("equipoise"), the tie goes to the veteran.

This is why the exact phrasing matters:

• •"is related to service" or "at least as likely as not" — meets the standard
• •"may be related" / "could be related" / "is possibly related" — does not. Speculative language has been repeatedly held insufficient to establish a nexus. "May" is not "is at least as likely as not."

A provider willing to support your claim but writing "may be related" out of clinical caution is unintentionally writing a letter the VA can discount. The standard does not ask for certainty — it asks for "50% or greater," which is a threshold many providers are comfortable stating once they understand that is the question being asked.

Strong letter vs. weak letter

The difference is reasoning. A rater can — and does — weigh medical opinions by the quality of their rationale.

The single most cited weakness in rejected medical opinions is the missing rationale. An opinion that states a conclusion without explaining the medical reasoning behind it can be assigned little or no probative weight — even with the right probability language. The mechanism paragraph — how the in-service event or primary condition produces the claimed condition — is the part that does the work.

For secondary and aggravation claims

A nexus letter for a secondary claim answers a slightly different question: whether the claimed condition was caused or aggravated by a service-connected condition (38 CFR § 3.310). After the May 2026 M21-1 update implementing Spicer v. McDonough, opinions supporting aggravation claims are evaluated on whether they reason through the but-for question — whether the worsening would have occurred absent the service-connected condition. See our Spicer guide for the details.

The bottom line

A nexus letter is the document that addresses the element most claims are missing. Its strength comes from a qualified signer, the correct legal standard ("at least as likely as not"), and — above everything else — a reasoned medical explanation specific to your history. A letter with those three things is evidence built to the standard raters apply. A letter without the rationale is a conclusion the VA is free to set aside.