Why am I getting a HTTP status code of 301 when I do an API request?

Starting April 18, 2016 the API servers are redirecting any http: requests to https. If you are making an API request using http, you should change it to https to avoid this status returned.

Do I need a license to run the RxNorm/RxTerms/Prescribable/RxClass/Interaction/RxCUI History API?

With one exception, no license is needed to use any of the above APIs. The only exception is the /rxcui/{rxcui}/proprietary function in the RxNorm API, which requires the user to have a valid UMLS license and pass a proxy granting ticket as a parameter to the function.

Is there a limit to the number of API calls I can make?

Yes, to accomodate the increasing usage and many users of the APIs, there is a limit on the number of API calls a user can make in a given time period. Consult the Terms of Service for more details.

How will I know if I exceed the terms of service limit?

If the API call limit is exceeded, then the servers will return an HTTP status code of 403 ("Forbidden").

Do the APIs allow Cross-origin resource sharing (CORS)?