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.
Neostigmine
Atropine
Morphine
Fentanyl