The Playbook › Step 2 of 7: Build your evidence
“Nexus” means link. A nexus letter is a written medical opinion that connects your current condition to your military service, or to another condition that is already service connected. It answers the third question in every claim: is this related to service?
VA never requires you to submit one. But when the link is not obvious from your records, a good nexus letter is often what decides the claim.
The short version
- The standard is “at least as likely as not”, which means a 50% or greater probability.
- A strong nexus letter shows the doctor reviewed your records and explains why.
- A one-line note saying “this is related to service” carries almost no weight.
- You do not need one for presumptive conditions.
- When evidence is evenly balanced, the law says the tie goes to the veteran.
On this page
The words that matter
VA does not need certainty. It needs a medical professional to say the connection is at least a coin flip. Opinions are usually written using one of these phrases:
| Phrase | Probability | Result |
|---|---|---|
| “Is due to” | 100% | Supports the claim |
| “More likely than not” | Greater than 50% | Supports the claim |
| “At least as likely as not” | 50% or greater | Supports the claim |
| “Less likely than not” | Less than 50% | Weighs against the claim |
| “Could be,” “may be,” “possibly” | Speculative | Usually given no weight |
A nexus letter that says your condition “might be related” to service does not meet the standard. The wording has to be clear.
What makes a nexus letter strong
Raters weigh medical opinions against each other. These are the things that make one persuasive:
- Qualifications. The writer states their credentials. A specialist in the relevant field carries more weight, but any licensed clinician can give an opinion.
- Record review. The nexus letter says which records were reviewed: service treatment records, post-service records, and prior VA exams.
- Accurate history. The facts in the nexus letter match your file. An opinion built on a wrong history can be thrown out.
- Rationale. This is the most important part. The doctor explains the medical reasoning: how the in-service event leads to the current condition, with reference to your records and, where useful, medical literature.
- The right standard. The conclusion uses “at least as likely as not” or stronger.
- Addresses the other side. If a C&P examiner gave a negative opinion, a strong nexus letter explains why that opinion is wrong or incomplete.
When you need one
- The condition was diagnosed years after service and your service records are thin
- You are claiming a secondary condition
- A C&P examiner gave a negative opinion and you are filing a Supplemental Claim
- VA denied for “no nexus” or no continuity of symptoms
- You are claiming aggravation of a condition that existed before service
When you probably do not
- Your condition is presumptive based on your service. See PACT Act, Agent Orange, and Gulf War.
- The condition was diagnosed in service and you have been treated for it continuously since.
- You are asking for an increase on a condition already service connected. That is about severity, not the link.
Who can write one
- Your own treating doctor, VA or private
- A specialist you see for the condition
- An independent medical examiner who reviews your file
Start with a doctor who already knows you. Some VA clinicians will write opinions and some will not. If you pay for an independent opinion, expect it to cost money up front, and be cautious about companies that sell letters in bulk or promise a result. A letter that reads like a template is easy for a rater to discount.
How to ask your doctor
Make it easy for them:
- Bring the relevant pages of your service and medical records
- Explain the “at least as likely as not” standard
- Ask them to explain their reasoning in a few sentences
- Offer a short written summary of your history for them to check
Many doctors are willing but have never been asked and do not know what VA is looking for.
What a nexus letter cannot do
It cannot stand in for the other two elements. You still need a current diagnosis and evidence that something happened in service. It also will not raise your rating. Ratings depend on how severe your symptoms are.
Next step
Pair the medical opinion with first-hand accounts. See buddy statements and your personal statement.
Free printable checklist. Everything to gather before you file, what to bring to your C&P exam, and what to check in your decision, on two pages.
Last reviewed October 2026. This guide is general educational information, not legal or medical advice, and this site is not affiliated with the Department of Veterans Affairs. Rules and rates change. Confirm details on VA.gov or with an accredited representative. See our disclaimer and editorial policy.