- Uncomfortable or moderately painful radiologic diagnostic and therapeutic procedures are being performed increasingly on outpatients. If sedation and analgesia are used, patients cannot drive themselves home or return rapidly to normal activities. This study compares the effect of fentanyl (100 micrograms), diazepam (7.5 mg), and placebo on driving ability of young volunteers as measured by the thacometer. Speed and accuracy were impaired at 30 and 120 minutes by both drugs, and by fentanyl more than diazepam. This study design may be suitable for the assessment of whether patients can drive safely after other analgesic drugs.