Field	Type	Required	Notes
`drug`	string	no	Drug or ingredient name, e.g. warfarin
`rxcui`	string	no	RxNorm concept id, as an alternative to `drug`
`source`	string	no	Restrict to one relationship source: ATC, ATCPROD, DAILYMED, FDASPL, FMTSME, MEDRT, MESH, RXNORM, SNOMEDCT, VA
`classTypes`	string	no	Comma-separated class types to keep, e.g. EPC,MOA. One of ATC1-4, CHEM, DISEASE, DISPOS, EPC, MESHPA, MOA, PE, PK, SCHEDULE, STRUCT, TC, VA
`includeProducts`	boolean	no	Also include classes carried only by branded or clinical drug concepts, not just ingredients
`limit`	number	no	Maximum classes, 1-100, default 50

Example input:

{
  "drug": "warfarin",
  "classTypes": "EPC,MOA,ATC1-4",
  "limit": 10
}

Metered

The 402 quotes a ceiling — the price of the declared cap, 100 classes — and settlement is for what the call actually used. A small request settles for less than it was quoted, so read the quote as an upper bound rather than a price.

Cost and provenance


Price	$0.003 per call, metered per classes
Source	NLM RxClass API (rxnav.nlm.nih.gov), a US federal work in the public domain
Licence	public-domain
Attribution	Drug class data from the NLM RxClass API

Why call it rather than answer from memory

Class membership is what a formulary, interaction or substitution decision actually turns on, and it is exactly the kind of fact a model will confidently invent. RxClass federates MED-RT, ATC, FDA SPL, VA NDF and SNOMED CT into one graph, and the raw feed returns a row per concept per class, so a caller asking about warfarin gets 202 rows for 56 distinct classes unless someone collapses them.

Calling it

A skill is loaded on its own, so this document has to be enough by itself. It does not assume the general fittings skill is also loaded. call.mjs ships alongside this file; if it is missing (this document reached you without the rest of the skill), fetch it once with curl -sO https://fittings.sh/skill/call.mjs.

Free, and needs no wallet. Read the schema and see what a call would cost:

curl -s https://fittings.sh/catalog/rxclass.drug-classes | jq

Paying. The endpoint answers 402 Payment Required with a signed quote, you sign an authorization, and a facilitator settles it — you never send a transaction and never pay gas. Before it spends anything, the client checks its own hash against the gateway and refuses rather than sign with logic the gateway has moved past:

# Dry run. Prints the quote and pays nothing, because no key is set.
node call.mjs rxclass.drug-classes drug=warfarin classTypes=EPC,MOA,ATC1-4 limit=10

# For real: a wallet holding USDC on Base, and nothing else.
export FITTINGS_PRIVATE_KEY=0x...
node call.mjs rxclass.drug-classes drug=warfarin classTypes=EPC,MOA,ATC1-4 limit=10

Spending is never a side effect of curiosity: with no FITTINGS_PRIVATE_KEY the client fetches the quote and stops. FITTINGS_MAX_PRICE caps what it will pay without asking, and defaults to $0.05.

Use a throwaway wallet holding a few dollars, not a main one. The key sits in plaintext wherever the agent runs, and the whole cost of entry is USDC on Base — no account, no API key, and no ETH, because the payer signs off-chain and the facilitator broadcasts and pays the gas.

Writing your own client instead of using this one? @x402/core and @x402/evm on npm, and the fittings skill records the two things that cost real debugging time: the challenge is in a response header rather than the body, and it must be re-signed from the exact request being retried.

If this is not the right service, https://fittings.sh/catalog/search?q=... searches all of them for free.

rxclass-drug-classes

RxClass drug class lookup

The call