Follow-up and followup are different spellings of the same word. The hyphenated form is more common, but the unhyphenated form is gaining ground. In either form, it works only as a noun or an adjective. When you need a verb, make it two words—follow up. For example, you might email a colleague to follow up on an earlier exchange, and your colleague might respond to your followup with a followup question.
A follow-up call from the mayor’s office asked whether they would host an event. [Columbus Dispatch]
The G2 is the follow-up to 2008′s G1, the first phone based on Google’s Android operating system … [WSJ Digits]
If these new cases follow the pattern of previous ones, prosecutors could follow up on regulatory actions with their own complaints. [New York Times]
By half-time Leinster led 13-6 and Munster were in contention to follow up their win against Connacht. [Irish Times]
We’ll add tweets below throughout the weekend, and follow up with more detail next week. [Guardian]