1)
2a) C16H21N3
2b) The first similarity is the two carbon chain with a nitrogen at the end (-C-C-N-). The second similarity is the -N-C-N- chain. The spatial arrangements of the flat 5-ring in histamine is also similar to the flat 6-ring in antihistamine.
With these similarities, antihistamine can then bind to histamine receptor sites on cells more tightly than histamine itself. This will block the effects of histamine and restricting any chemical messengers to be produced by histamine.
3)
4a) In Singapore, i believe that Supplement Facts as shown above is available outside the bottle to see the content of the tablet. This address identification and quantification of the active ingredient. Furthermore, it bold the important sentence-If you are pregnant, nursing, taking any medication or have a medical condition, please consult your healthcare practitioner before taking any dietary supplement. By doing so, it address the issue that the herbal remedy may or may not have side effect with another alternative or prescription medicine, which could mean consume at your own risk. Lastly, on the some label, there will have some logos as shown below. Having GMP logo, it meant that the manufacturers have good manufacturing practice which i think will address the quality control in manufacture.
4b) Under Singapore’s legislation, general provisions as to dealing with medical products (section 5) and provisions as to manufacturing and wholesale dealing (section 6) apply on herbal remedy as well. Section 8 where there is exemption of herbal remedies (can ignore the restrictions on section 5 & 6) apply if certain conditions are met.
Singapore Statutes Online, July 2016. LICENCES AND CERTIFICATES RELATING TO MEDICINAL PRODUCTS .Taken from: http://statutes.agc.gov.sg/aol/search/display/view.w3p;ident=24a667bc-38a1-4145-ad95-786452db0c36;page=0;query=DocId%3Ace1dda35-2048-4872-917f-65899143140d%20%20Status%3Ainforce%20Depth%3A0;rec=0#pr8-he-.