"Recently, I smoked a fair amount of marijuana (about 3-4 hits), and then proceeded to smoke some more about 3 hours later. After two hits, I grew extremely tired, almost like I was going to pass out. My vision took on a red tint and began to obscure, almost to the point of blindness; however, upon drinking a glass of water, my sight returned and the red tint went away almost instantly, although I remained shaky and extremely nervous for the remainder of the night. I did not consume any other drugs besides marijuana that night or the night before. Does anyone know what could have happened to me? Was I in any sort of danger? How can I avoid having this happen to me again? Any information would be greatly appreciated, my thanks in advance."
What you describe sounds like the beginning of a "fainting spell" from which you recovered before the actual faint. In otherwise healthy individuals this is usually a transient episode of low blood pressure which resolves on its own when the symptoms force the patient to sit or lie down. Dehydration is often a contributing factor, and drinking water was probably an excellent idea. Cannabis itself has no consistent effect on blood pressure, but once the mind and emotions enter the picture changes in heart rate and other significant cardiovascular responses can happen secondarily. If you do not have any current health problems and are not taking any medications I would not expect this to happen again. If it does, I would see your physician for evaluation.