Secondary Service Connection

A secondary claim asks VA to service connect a condition that did not start in service, because it was caused or worsened by a condition that is already service connected. The authority is 38 CFR §3.310. It is a short regulation that decides an enormous number of claims, and most veterans who are denied under it are denied for a reason they could have anticipated.

What §3.310 actually says

The regulation has two working parts, and they are different legal theories with different evidence requirements.

• §3.310(a) — causation. A disability that is proximately due to or the result of a service-connected disease or injury shall be service connected. In plain terms: the service-connected condition brought the new one into existence.
• §3.310(b) — aggravation. Any increase in severity of a non-service-connected disease or injury that is proximately due to a service-connected disability will be service connected — but only for the degree of disability over and above the baseline level of severity that existed before the aggravation. The new condition was already there. Service connection attaches to the extra harm.

The aggravation prong existed in case law before it was codified — see Allen v. Brown, 7 Vet. App. 439 (1995) — and it is regularly overlooked. Veterans plead causation, VA finds the condition pre-existed the service-connected disability, and the claim dies without anyone reaching the question that would have won it. We cover that prong in depth on the aggravation page.

The three elements of a secondary claim

1. A service-connected primary condition. It must already be service connected, or granted in the same decision. A condition you believe should be service connected but is not yet cannot support a secondary claim — which is why sequencing matters, and why a denied primary often needs to be fixed first.
2. A current diagnosis of the secondary condition. Symptoms are not a diagnosis. VA needs a recognised diagnosis in the medical record during the claim period.
3. A medical nexus between the two. Competent medical evidence that the primary condition caused or aggravated the secondary one.

What a sufficient nexus opinion looks like

The legal threshold is the benefit-of-the-doubt standard in 38 USC §5107(b) and 38 CFR §3.102: if the evidence for and against is in approximate balance, the veteran wins. That translates into the phrase VA reviewers look for — at least as likely as not, meaning a 50 percent or greater probability. Not "possible." Not "could be related." Not "may have contributed."

A usable opinion generally has four features:

• The standard, stated correctly. "It is at least as likely as not that the veteran's obstructive sleep apnea is caused or aggravated by his service-connected PTSD."
• Both theories addressed. Causation and aggravation. An opinion that only says "not caused by" leaves the aggravation prong undecided, and an undecided prong is an incomplete adjudication.
• A rationale. The medical reasoning, in the examiner's own words, tied to this veteran's records. A conclusion without reasoning carries almost no evidentiary weight, whichever direction it points.
• A stated evidentiary basis. That the provider reviewed the claims file or the relevant treatment records, and which ones. An opinion written from a single visit, with no record review, is easy for VA to discount.

We go through the anatomy of a strong opinion in detail on the nexus letter page. The short version: the rationale is the letter. Everything else is formatting.

The medication pathway

One of the most under-used theories in the whole system: a disability caused by the treatment for a service-connected condition is compensable on a secondary basis, because the treatment is a consequence of the service-connected disability.

• NSAIDs for a service-connected musculoskeletal condition. Years of high-dose ibuprofen, naproxen or meloxicam for a service-connected lumbar spine or knee disability, followed by a GERD or gastritis diagnosis, is a recognisable and well-documented pathway.
• Psychotropic medication. Several classes of psychiatric medication are associated with weight gain and with disturbed sleep architecture. Where a service-connected mental health condition is treated with them, and weight gain or a sleep disorder follows, that is a chain worth pleading explicitly.
• Sexual dysfunction from SSRIs. A very common and very commonly unclaimed consequence of treatment for service-connected depression, anxiety or PTSD.

Pleading a medication pathway means naming the drug, the prescribing indication, and the date range in the claim itself. If the examiner has to go looking for it, they usually will not.

The pathways we see most often

None of these are automatic. Each one is a medical question that has to be answered on the facts of your record. What the table gives you is the link the opinion has to explain.

Our conditions library goes condition by condition, with the rating criteria and the secondary pathways that attach to each.

Erectile dysfunction and SMC-K

Erectile dysfunction is worth understanding on its own, because it is frequently granted and frequently misunderstood. Under the rating schedule, ED without deformity of the penis usually carries a 0 percent rating. Veterans see the zero and assume the claim failed.

It often did not. Service connection for ED can support special monthly compensation under 38 CFR §3.350(a) — SMC-K, for loss of use of a creative organ. SMC-K is a fixed monthly amount paid in addition to your regular compensation, and it is not subject to the combined rating table. A 0 percent rating with SMC-K is a real award.

Why secondaries get denied

• No medical opinion in the file. The single most common reason. A personal statement is competent evidence about symptoms, but a veteran is generally not competent to establish medical causation.
• A negative C&P opinion with no rationale. "Less likely than not, as there is no medical literature supporting this association" is a conclusion, not reasoning — and where the literature does exist, that sentence is also wrong. See inadequate C&P exams.
• Only one prong addressed. The examiner answers causation and never reaches aggravation. That is an incomplete opinion, and an incomplete opinion is a recognised basis for further review.
• The chain was not pleaded. Where the real pathway runs through an intermediate step — most often weight gain — and nobody said so, the examiner evaluates a link that was never the argument. See obesity as an intermediate step.
• The primary is not service connected yet. The secondary claim has nothing to attach to and is denied on the law.

What to do after a denial

A denied secondary claim is one of the more workable denials in the system, because the missing piece is usually identifiable from the decision itself. Read the reasons and bases section and find the sentence that decided it.

• New medical evidence, including a nexus opinion? That is a supplemental claim, which requires new and relevant evidence.
• The evidence was already there and VA got it wrong? That is a higher-level review — no new evidence permitted, a senior reviewer looks again at the same record.
• The exam was inadequate or a prong went unaddressed? That can be argued as a duty-to-assist error in HLR, or taken to the Board.

Choosing between those lanes changes how long you wait and what evidence you are allowed to submit. We break the choice down in HLR vs supplemental claim, and the effective date consequences in effective dates and back pay.

The law here is moving

Two developments matter for anyone filing a secondary claim now. The Federal Circuit's decision in Spicer v. McDonough adopted a but-for causation standard for secondary service connection, and VA folded it into the M21-1 in May 2026 — which produced a widely shared and completely false rumour that VA had eliminated secondary claims. It has not. We deal with that directly on the Spicer page. VA also updated its exam-necessity guidance for aggravation claims at M21-1 IV.i.1.B at the same time.

How we build these claims

A secondary claim is an argument, not a form. We identify which of your service-connected conditions can carry a secondary, work out whether the theory is causation, aggravation, or both, and make sure the medical opinion answers the question VA is actually going to ask — including every link in the chain where there is more than one. Where an examination has already gone against you, we look at whether it was adequate before we look at anything else.

If you have already been denied, start with what to do when a VA claim is denied. If you want to see what a grant would be worth alongside your existing ratings, the combined ratings calculator will show you how the numbers stack.