Is a moderate to severe blockage in left carotid artery dangerous?

Carotid blockage. If asymptomatic with use of proper medical therapy a moderate carotid blockage is fairly low risk. Many vascular surgeons would favor no intervention in this situation. If symptomatic, would recommend repair of this. Options include stenting or open surgery, carotid endarterectomy. Best data suggests that stenting over age 65 -70 has higher risk of stroke than endarterectomy.
Depends on how bad. Moderate narrowing less than 75-80% not very dangerous, low risk of stroke. More than this % of narrowing has risk stroke of 1-3% per year, if no symptoms.
Carotid. Having a significant carotid lesion increases the chance of having an episode related to it. Generally we treat with anti atherosclerotic meds and aspirin. Sometimes we do prophylactic carotid endarterectomy if the clinical situation supports that step.