Constipation is caused by all of the following drugs EXCEPT :

Neostigmine is an inhibitor of acetylcholinesterase and thus acts like a cholinergic drug. Therefore, it can produce diarrhea (not constipation). Atropine is an anticholinergic drug, thus can cause constipation. Morphine and fentanyl are opioids. These can also result in constipation.