Since 2004 the United States has collected approximately $8 billion from fraud enforcement actions against pharmaceutical manufacturers accused under the federal False Claims Act of illegally promoting drugs for off-label uses. Using the case of gabapentin (Neurontin), a drug approved for epilepsy but prescribed for a variety of conditions, we sought to determine whether the enforcement action also influenced off-label prescribing rates. We conducted a segmented time-series analysis using key legal milestones: the initiation of a sealed investigation, public announcement of the investigation, and settlement of the case. Off-label use grew steadily until settlement, when gabapentin prescriptions declined for both off-label and on-label indications. Because enforcement actions targeting illegal off-label promotion might not have a substantial deterrent effect on prescription rates until after settlement, they should be combined with other efforts to combat off-label promotion. These could include additional resources for enforcement and a steep increase in penalties because settlements to this point have been dwarfed by the financial gains to pharmaceutical companies from engaging in improper off-label marketing.