Lorazepam is a drug that has been widely used for over 30 years. Whereas its therapeutic and amnestic effects are fairly well known, the visuo-perceptual impairments induced by this drug have been studied to a much lesser degree and only little is known about the influence of lorazepam on the time course of visual information processing. To gain a better insight specifically on these temporal characteristics, we used the recently discovered backward masking technique, 'shine-through', in which a vernier target precedes a grating. We tested subjects, treated with lorazepam, diazepam, or a placebo, with masking gratings of various spatial layouts. Our experiments reveal surprising results. First, for the unmasked vernier target, lorazepam induced a strong deterioration of performance compared to both diazepam and placebo. Performance deteriorated even more significantly if a masking grating was presented following the vernier. We observed that vernier discrimination could be completely abolished even if the grating appeared 400 ms after the vernier presentation. Such long time intervals are beyond usual visual masking effects. When performing the task under placebo, the participants perceived the vernier target and the masking grating as two independent time events rather than as a single event. It appears that lorazepam prolongs dramatically the processing of visual targets. The masking effects revealed here are specific to the type of grating and are much stronger under lorazepam than under diazepam.