I actually tried the chemist in the Siam area and had no success, go to Nana as there is plenty of Kamagra for sale on the street and in the Pharmacies
Not all chemists do prescription medicines you need to find a pharmacy, as the chemists normally just do the retail stuff.
Siam is great as I’ve stayed at the Pathumwan Princess Hotel MBK and the Mercure Siam which is an easy walk to all the shopping malls in the area,