Perhaps something like nicorette gum or patches could help slowly ease them off. This may reduce their symptoms because the body is still receiving the drug in small doses.
All of it. Because if you have a open wound and the other person does. You are sharing blood. The second one is through someone donating there blood. Needle stick injuries is if the person uses the same needle on different patients.
All of these but the nosebleeds seem correct, but something much stronger like cocaine can cause nosebleeds.
But if you want just ONE answer 'impaired memory and learning' seems like the best one to pick because that is one of the most noticeable signs of marijuana use.
This website talks about the symptoms of use, I trust its accuracy:
https://www.webmd.com/mental-health/addiction/marijuana-use-and-its-effects#1
