Secondary Service Connection: When One Disability Causes Another

What is secondary service connection?

Secondary service connectionis a way to get a disability rated when the disability itself didn't start in service — but a condition that is already service-connected caused it or made it worse. The rule lives in 38 CFR § 3.310: a disability that is proximately due to or the result of a service-connected condition is treated as service-connected too.

You do not have to tie the secondary condition back to an in-service event directly. The link runs through the already-established condition. If your knee is service-connected and the altered gait it caused wore out your hip, the hip can be claimed secondary to the knee — even though nothing happened to your hip in uniform.

Three ways a condition can be secondary

§ 3.310 recognizes more than one path. A condition can qualify as secondary through:

1. 1Causation. The service-connected condition directly caused the new one — e.g., service-connected diabetes causing peripheral neuropathy.
2. 2Aggravation. The new condition existed already, but the service-connected condition made it permanently worse. The VA rates the additionaldisability beyond the condition's natural baseline.
3. 3Treatment interference. The medication or treatment for a service-connected condition caused a new problem — e.g., long-term NSAIDs for a service-connected joint causing gastrointestinal damage.

The Spicer "but-for" causation standard

In Spicer v. McDonough, 61 F.4th 1360 (Fed. Cir. 2023), the Federal Circuit clarified what "proximately due to" means for causation under § 3.310. The court adopted a "but-for" framing: the question is whether the secondary condition would have occurred but for the service-connected condition.

The VA folded this into the M21-1 adjudication manual in its May 1, 2026 update, so raters now evaluate secondary causation claims against the but-for test. In practice this sharpens what a supporting medical opinion needs to say: not just that the conditions coexist, but that the service-connected condition is what set the secondary one in motion.

Common secondary chains

Some primary-to-secondary links show up again and again. If you're service-connected for the condition on the left, the condition on the right is worth investigating:

These are illustrations, not a checklist — the right secondary claim depends entirely on your own medical picture. The point is that a single service-connected condition often opens the door to several legitimate secondary claims you may not have connected on your own.

How to file a secondary claim

A secondary claim is filed the same way as any disability claim — on VA Form 21-526EZ— but the framing is different. You're telling the VA the new condition is caused by or aggravated by an existing service-connected one:

• Name the new condition and clearly state it is secondary to the specific condition you're already rated for.
• Establish the current diagnosis of the secondary condition — symptoms alone are not enough.
• Provide a medical nexus opinion linking the secondary condition to the primary one, addressing the but-for causation standard.
• Include treatment records that show the timeline — the primary condition came first, the secondary followed.

Because the anchor condition is already established, secondary claims can be among the most winnable claims a veteran has — provided the medical link is documented clearly.

How VA Claim Commander identifies secondary opportunities

The hardest part of a secondary claim is often just noticing it exists. VA Claim Commander reads your records and rating history with secondary chains in mind:

• It maps your already service-connected conditions and flags the secondary conditions most commonly linked to each.
• It scans uploaded records for diagnoses that may be downstream of a condition you're already rated for.
• It frames the documentation around § 3.310 and the but-for causation standard so a provider's opinion targets what raters look for.
• It builds a nexus-letter starting point that connects the secondary condition to its primary anchor — for your provider to review and sign.

This article is educational information about the VA claims system — it is not legal or medical advice, and it does not predict or promise any claim outcome. Rating decisions are made solely by VA adjudicators based on the evidence in each veteran's file. VA Claim Commander is a self-service documentation tool, not a VSO, law firm, or VA-accredited representative, and it does not provide medical opinions — every nexus opinion requires independent review and signature by a licensed clinician.