Fact-checking has become an important feature of the modern media landscape. However, it is unclear what the most effective format of fact-checks is. Some have argued that simple retractions that repeat a false claim and tag it as false may backfire because they boost the claim's familiarity. More detailed refutations may provide a more promising approach, but may not be feasible under the severe space constraints associated with social-media communication.In two experiments, we tested whether (1) simple 'false-tag' retractions can indeed be ineffective or harmful; and (2) short-format (140-character) refutations are more effective than simple retractions. Regarding (1), simple retractions reduced belief in false claims, and we found no evidence for a familiarity-driven backfire effect. Regarding (2), short-format refutations were found to be more effective than simple retractions after a 1-week delay but not a one-day delay. At both delays, however, they were associated with reduced misinformation-congruent reasoning.