Generally, no hyphen is required to link this prefix to root words. In the case of many prefixes, the element is initially attached to the root word, but as readers and writers become accustomed to the new construction, the linking device is omitted, and the components are joined.
However, some constructions resist this transformation because the resulting fusion looks odd. We make exceptions for certain contiguous vowels (anti-inflammatory) or for some awkward-looking combinations (pro-choice, thoughproactive and most other pro- constructions are closed). But the default setting, at least in American English, is to discard the hyphen and close up the resulting space.
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