Yes. Lean is dangerous because of the effects caused by the two main ingredients in the cough syrup typically used to make lean: promethazine, a central nervous system (CNS) depressant, and codeine, a respiratory depressant.
Respiratory depressants are particularly dangerous, as they can prevent oxygen from entering the lungs and cause carbon dioxide to build up in the bloodstream. Too much carbon dioxide in the bloodstream can cause life-threatening symptoms like:
Stopped breathing (respiratory arrest)
Heart attack
Brain damage due to insufficient oxygen
Slow heart rate
Coma
Death