A nexus letter is a written medical opinion from a physician that links a current diagnosed condition to an in-service event, injury, or disease. The word "nexus" means connection — the letter establishes the legal bridge between what happened in service and what is wrong today.
The VA is required to obtain a C&P examination when one is needed, but those exams are often brief and the opinions inadequate. A well-written private nexus letter from your own treating physician — or an independent medical expert — can be the single most important document in a disability claim.
What a Nexus Letter Must Contain
Current Diagnosis
The letter must identify a current, diagnosed condition. A history of symptoms without a formal diagnosis is not sufficient. The diagnosis should use standard medical terminology and ideally reference the ICD code or DSM criteria where applicable.
In-Service Incident or Exposure
The letter must reference the specific event, injury, occupational exposure, or disease that occurred during the Veteran's active military service. This can include a documented incident, a pattern of duties (e.g., noise exposure from weapons or aircraft), or a documented illness. The physician should acknowledge the Veteran's account of in-service events as part of the medical history.
The Opinion Itself — The Magic Words
The opinion must state one of the following: • "at least as likely as not" (50% or greater probability) — the standard for direct service connection • "more likely than not" (greater than 50%) — stronger, but not legally required • "less likely than not" — a negative nexus The phrase "at least as likely as not" meets the legal threshold. The letter does not need to say the condition is "definitely" or "certainly" service-connected.
Rationale — Not a Conclusion
The most common reason nexus letters are rejected is that they are conclusory — they state an opinion without explaining why. The VA requires that the physician provide reasoning. This means citing medical literature, describing the mechanism of injury, explaining the natural history of the condition, or referencing the Veteran's treatment records. "It is my opinion that X is related to service" alone is legally inadequate. "It is my opinion that X is related to service because [medical reasoning]" is adequate.
Physician Credentials
The letter should be on the physician's letterhead, signed, and include their medical license number or specialty credentials. The VA weighs the opinion of a relevant specialist (e.g., an orthopedic surgeon opining on a knee condition) more heavily than a general practitioner on the same topic, though both can provide valid nexus opinions.
When You Need a Nexus Letter
Not every claim needs a private nexus letter — some conditions are presumptively service-connected (certain cancers, Agent Orange conditions, Gulf War illness, etc.) or are clearly documented in service records. The following situations almost always benefit from a private opinion:
Denied already? See the Appeals Roadmap →
Secondary Service Connection
A condition caused or aggravated by an already-rated service-connected condition. Example: sleep apnea secondary to PTSD, depression secondary to chronic pain, kidney disease secondary to hypertension. The nexus must link the secondary condition to the primary SC condition.
Delayed-Onset Conditions
Conditions that develop years after service, such as hearing loss, certain cancers, or degenerative joint disease. The nexus must explain why the in-service exposure or injury is still a cause despite the time gap.
Mental Health Secondary Claims
Depression, anxiety, and adjustment disorder claimed as secondary to physical service-connected conditions (e.g., PTSD secondary to MST, depression secondary to chronic pain). A psychiatrist or psychologist is the appropriate expert.
Sleep Apnea
Sleep apnea is rarely diagnosed during service but is commonly filed years later. A nexus linking sleep apnea to in-service events (weight gain from inactivity during recovery, PTSD-related sleep disruption, or in-flight oxygen issues) requires a specific medical rationale.
Conditions Not in Service Records
If your condition was not diagnosed or treated during service but you have a credible account of symptoms starting in service, a nexus letter can bridge that gap — especially combined with a lay statement about in-service symptoms.
Toxic Exposure / Environmental Claims
Burn pit exposure, contaminated water (Camp Lejeune), Agent Orange, radiation exposure. Even with presumptive eligibility, some conditions outside the presumptive list need a nexus opinion tying the specific condition to the specific exposure.
After a Denial
If the VA denied service connection and cited an inadequate or negative C&P exam, a private nexus letter from a qualified expert is often the most effective way to reopen the claim or support an appeal. See the Appeals Roadmap (/appeals) for the three review options.
When the C&P Examiner Got It Wrong
A C&P examiner's negative opinion can be rebutted by a private nexus letter. The VA must weigh both opinions. If the private letter is better reasoned and more detailed, it can outweigh the C&P opinion even though the examiner was VA-contracted.
Getting an Adequate Letter from Your Doctor
Provide Your Records First
Give your doctor your service records (DD-214, service treatment records), your VA rating decision, and your relevant private treatment records before the appointment. A physician cannot write a credible nexus without reviewing your history. Never ask a doctor to write an opinion without giving them the underlying facts.
Ask for the Specific Language
Tell your doctor you need a letter stating whether your condition is "at least as likely as not" related to your military service, and that the letter must include their medical reasoning — not just a conclusion. Many physicians are unfamiliar with VA terminology and will write a letter that is inadequate until guided.
Independent Medical Experts (IMEs)
If your treating physician is unwilling or unqualified to write a nexus, consider hiring an independent medical expert. Several companies and individual physicians specialize in VA nexus opinions. Costs typically range from $500–$2,000 depending on complexity. For high-value claims (TDIU, 100% schedular), this investment often pays off.
What Makes a Letter Inadequate
The VA can legally discount a nexus letter if it: (1) provides no rationale, only a conclusion; (2) was based on an inaccurate medical history; (3) was written without reviewing service records; or (4) uses language like "possibly" or "might be" rather than the legal standard "at least as likely as not." A letter that says "it is possible" that the condition is related to service does not meet the legal threshold.
Bring This to Your Doctor's Appointment
Checklist — bring with you:
- Your DD-214 and service treatment records
- Your most recent VA rating decision letter (if you have one)
- Relevant private treatment records for the condition
- A note on who the right specialist is (e.g., orthopedist for a joint, psychologist or psychiatrist for a mental-health condition)
Talking points — what to ask for (you can read this aloud):
“Please state in a letter whether my [condition] is ‘at least as likely as not’ related to my military service, and include your medical reasoning— not just a conclusion — on your letterhead with your credentials and signature.”
A letter that says only “it is possible” or gives an opinion with no reasoning does not meet the VA's legal standard. If your condition was caused by another service-connected condition, ask the doctor to link it to that condition (secondary service connection).