1a) C5H9N3

1b)  NH2 from this histamine structure

1c) The nitrogen in Amine (NH2) makes it water-soluble

2a)  C16H21N3

2b) Because of the similar aromatic rings in both of these structures,  allowing anti-histamine to compete with histamine

3) Aspirin, morphine, amphetamine, estrogen falls under produce a physiological response in the body while penicillin and Keflex antibiotic under inhibit growth of substances that cause infections

4a) The herbal supplement manufacturers need to be honest in their product by putting labels to support their claim that the products are safe for consumption, explaining the functions of the ingredients listed, whether or not these ingredients may or not be suitable to certain groups of people that might get allergic reactions from it.

4b) In Singapore, under Herbal/alternative it says that it is not subject to approvals licensing or registration before sold locally, they need not be licensed by HSA, dealers are accountable for ensuring information is truthful. Under OTC medicines/ prescription, they are subject to checks and product registration before sold and must be licensed by HSA. The difference in regulations may be due to herbal medicines containing typically ingredients from natural source

 

Image reference:

 https://www.reddit.com/r/Drugs/comments/sn54c/i_thought_you_guys_would_like_this_wallpaper/

http://www.celestialroots.com/histamine.php