It’s the result of a compromise with the FDA. The FDA insists that apps that use Dexcom’s API cannot be used to make treatment decisions.
Data Availability
Data from the Dexcom API is available with a three-hour delay. This delay is enforced on the data upload time, not on the timestamps of individual data points. The G5® and G6® Mobile applications upload once per hour, so the data will (on average) be 3.5 hours delayed. On the other hand, data uploaded directly from a receiver over USB (via the Dexcom CLARITY® uploader) is available immediately because it is viewed as an active, rather than passive, upload. For more information on this delay, please see the FAQ.
Dexcom API | Home
and
See “Can apps get real-time estimated glucose values (EGVs) with the Dexcom API?”
I believe Dexcom only supports Apple Watches officially for real time data using their app. Pebble support was done through a hack (NightWatch or NightScout) or if I remember correctly, required a “Dexcom → phone → cloud → back to phone → pebble” communication path and that path used Dexcom’s internal APIs not the public API.
I’m posting a tutorial on the OAuth part. Stay tuned.