Background Nonsteroidal anti-inflammatory drugs are widely used for migraine, but gastrointestinal tolerability limits use. We previously reported results from the first treatment period of this 2-period, randomized, controlled study comparing DFN-15—an oral, ready-made liquid solution of a selective cyclo-oxygenase-2 inhibitor celecoxib—with placebo for the acute treatment of a moderate-severe migraine attack. Herein, we report the effects of treatment for the second treatment period. Methods In the first treatment period of this trial, adults with migraine were randomized to double-blind trial treatment of attacks of moderate or severe pain with DFN-15,120 mg or placebo. For the second treatment period, reported herein, participants were re-randomized to treat an attack of any baseline pain intensity (mild, moderate, or severe). Co-primary efficacy endpoints specified for the first attack were not specified for the second attack. Results Of the 531 patients who completed the first treatment period, 491 (n = 243 DFN-15; n = 248 placebo; 87% female, mean age 41 years) were re-randomized into the second double-blind treatment period. Baseline pain intensity was mild in 17.2% (85/493) of patients, moderate in 58.4% (288/493) of patients, and severe in 22.9% (113/493) of patients. At 2 hours post-dose, DFN-15 was superior to placebo for freedom from pain (46.2% [110/238] vs 31.1% [76/244], p ≤ 0.001) and the most bothersome symptom (63.4% [121/191] vs 50.0% [98/196], p = 0.010). Treatment-emergent adverse events (TEAEs) occurred in 7.1% (35/493) of patients (DFN-15: 6.1% [15/244]; placebo 8.0% [20/249]). Study drug-related TEAEs occurred in 5.1% (25/493) of patients (DFN-15: 4.5% [11/244]; placebo 5.6% [14/249]); nausea (1% [5/493]) and dysgeusia (0.8% [4/493]) were most common. No serious TEAEs, severe TEAEs, or TEAEs leading to study drug termination were reported. Conclusions DFN-15 was superior to placebo for pain freedom and freedom from the most bothersome symptom when patients treat a migraine attack of any baseline pain intensity. Rates of TEAEs did not differ between treatment groups.