Background Lacosamide (LCM) is a novel antiepileptic drug (AED) with potential benefit as adjunctive treatment in patients with partial‐onset seizures. As yet, limited information on cognitive effects of LCM is available, especially in real‐life settings. Aims In this open clinical prospective study, the cognitive effects of LCM were evaluated when used as adjunctive antiepileptic therapy in patients with refractory epilepsy. Methods We included 33 patients aged between 16 and 74 years (mean: 37 years). All patients had a localization‐related epilepsy. Patients were assessed at baseline before starting LCM treatment and during follow‐up when the optimal clinical dose was achieved. Materials Subjective complaints were evaluated using the SIDAED; effects on cognition were evaluated using the computerized visual searching task (CVST). Results The CVST showed significant faster information processing reaction times at the second evaluation (P = 0.013), which was not correlated with seizure control, type of epilepsy, age, gender, drug load, number of concomitant drugs, dose or duration of LCM treatment. On the SIDAED, patients complained more about their cognitive function at the second evaluation (P = 0.005). For the SIDAED, a positive correlation at follow‐up was found between the total severity score and higher age (r = 0.375, P = 0.031), but not with epilepsy factors or treatment characteristics. Discussion/Conlusion Screening of the cognitive effects of LCM showed that LCM does not have negative effects on information processing speed. As this is the most sensitive function for cognitive side effects of AEDs, LCM does not seem to induce the common negative cognitive effects. Remarkably, patients complained more, especially about their cognitive function, which is possible the ‘doing better, feeling worse phenomenon’.