How many different chemical compound names are there in the 18 M entries?
Having lots of data is good, but I don’t think training a tokenizer would help you unless the words you are interested in are frequent enough to be selected for the tokenizer’s vocabulary.     I’m not sure how the tokenizer chooses its vocabulary, but word-frequency must be important.   I’m guessing that “medical drug data” would still include lots of normal-English words, many of which would be more frequent than the chemicals.
[I am not an expert].