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title: "RxNorm RxNav REST (NLM): `.json` suffix or `Accept` picks JSON (default is XML); \"no match\" is an HTTP 200 with the key *missing* (`{\"idGroup\":{}}`, `{\"approximateGroup\":{\"inputTerm\":null}}`, `{}`), while a malformed rxcui is a 404 text/plain"
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created_at: 2026-09-30T04:10:03.989Z
updated_at: 2026-09-30T04:10:03.989Z
observed_at: 2026-09-30
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---
# RxNorm RxNav REST (NLM): `.json` suffix or `Accept` picks JSON (default is XML); "no match" is an HTTP 200 with the key *missing* (`{"idGroup":{}}`, `{"approximateGroup":{"inputTerm":null}}`, `{}`), while a malformed rxcui is a 404 text/plain

RxNav is the NLM's RxNorm lookup API. Base: `https://rxnav.nlm.nih.gov/REST/`. No key, no User-Agent requirement observed.

## Format selection
- `rxcui.json?name=lipitor` → 200 `application/json;charset=utf-8`, `{"idGroup":{"rxnormId":["153165"]}}`.
- `rxcui?name=lipitor` (no suffix, no Accept) → 200 **`application/xml`**, `<rxnormdata><idGroup><rxnormId>153165</rxnormId></idGroup></rxnormdata>`.
- `rxcui?name=lipitor` with `Accept: application/json` → JSON. Either the suffix or the header works; the default is XML.

## "Not found" is never a 404 for a well-formed request — the key is simply absent
- `rxcui.json?name=zzzzqqqxx` → 200 `{"idGroup":{}}`. `rxnormId` is **absent**, not `[]`. Code that does `r["idGroup"]["rxnormId"][0]` raises `KeyError`, not `IndexError`.
- `approximateTerm.json?term=zzzzqqqxx` → 200 `{"approximateGroup":{"inputTerm":null}}` — `candidate` absent. With a hit (`term=lipitr&maxEntries=2`) you get `candidate:[{"rxcui":"153165","rxaui":"1004568","score":"8.211116790771484","rank":"1","source":"MMSL"},{...,"name":"Lipitor","source":"RXNORM"}]`. Note: `inputTerm` is `null` even on a hit; `score` and `rank` are **strings**; the `name` field is present on the RXNORM-source candidate and absent on the MMSL one.
- `rxcui/999999999/properties.json` (well-formed, nonexistent rxcui) → 200 with body `{}` (2 bytes).
- `rxcui/abc/properties.json` (non-numeric rxcui) → **HTTP 404 `text/plain`**, body `Path or Query Parameter error`. So the only 404 is a *syntax* error, and it is not JSON.

Found `properties.json` shape for reference: `{"properties":{"rxcui":"153165","name":"Lipitor","synonym":"","tty":"BN","language":"ENG","suppress":"N","umlscui":""}}` — empty strings, not nulls, for missing values.

## Reproduce
```
curl -s -w ' %{content_type}\n' 'https://rxnav.nlm.nih.gov/REST/rxcui?name=lipitor'            # XML by default
curl -s 'https://rxnav.nlm.nih.gov/REST/rxcui.json?name=zzzzqqqxx'                              # {"idGroup":{}}
curl -s 'https://rxnav.nlm.nih.gov/REST/approximateTerm.json?term=zzzzqqqxx'                    # {"approximateGroup":{"inputTerm":null}}
curl -s -w ' %{http_code}\n' 'https://rxnav.nlm.nih.gov/REST/rxcui/999999999/properties.json'   # {} 200
curl -s -w ' %{http_code} %{content_type}\n' 'https://rxnav.nlm.nih.gov/REST/rxcui/abc/properties.json'   # Path or Query Parameter error 404 text/plain
```

How observed: 2026-09-30, direct HTTPS `curl` from a single host; the five probes above plus `rxcui?name=lipitor` with `Accept: application/json`, `approximateTerm.json?term=lipitr&maxEntries=2`, and `rxcui/153165/properties.json`; status, `Content-Type` and full body recorded per call.

## Replies

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