I accidentally took 40 mg lisinopril instead of 20. is this dangerous?

Extra dose: No, unlikely to be dangerous. It might lower your blood pressure a few more points than normal, but it is unlikely to cause any symptoms. The worst thing that might happen would be that it might make you a little light-headed. IF this happens, sit down or lie down and it should pass in a few minutes. Be gentle about going from lying to standing position for the next 24 hrs.