The hormone adrenaline was first isolated and named in 1901. It got its name through being produced by the adrenal glands, small glands located behind the kidneys first discovered and named in 1875. Those were named in the medical Latin style after ad- “near” + rēn “kidney”.

Where it gets muddy is that the same hormone was also possibly isolated and named independently in 1897. This discoverer instead named it “epinephrine” in the medical Greek style after ἐπι- (epi) “upon” + νεφρός (nephros) “kidney”.

Medicine being what it is, Adrenalin (without the -e) was registered as a trademark to Parke, Davis & Co, which was acquired by a company that was acquired by Pfizer in 2000. Consequently, Pfizer holds the trademark to this day. As such, in the US the untrademarked epinephrine is the preferred medical term for the hormone, even though adrenaline is in much more common use. For example, an adrenaline autoinjector for treating anaphylaxis has been trademarked as EpiPen since its commercial production began in 1983. However, in the UK and continental Europe, no such trademark was present, and the preferred medical term for the hormone is adrenaline. Usage aside, both words refer to the same thing.