A few observations on the psychology involved in dealing with a pharmacy in Thailand. In my experience, female pharmacists are far more likely to accommodate you than a male. Just as their are prigs in our home countries, there are prigs in Thailand who are not going give out a medicine needing an RX which I think even a Z-pak and Doxycycline need, technically, in LOS.
However, I have gotten both mainly by going in and describing my symptoms or making up my symptoms and advising the pharmacist which antibiotic has worked for me in the past. A Z-pak is very easy to come by and will cure chlamydia and I would strongly recommend either buying the doxycycline in Thailand or through one of the legitimate online companies that offers meds. via an MD consultation. A previous poster is correct that often these meds do not seem to have the potency of an actual med. bought back home in Aus. or the US.
A note to Americans, I will get a Doxy. rx. from my MD by telling her that I am traveling to a malarial zone and don't react well to Malarone, the other front-line malarial medicine. Doxy. is a first-line treatment for Malaria and a script for that reason is not hard to come by in the US. Bad news is that the price which used to be next to nothing has been jacked up to outrageously high standards b/c Doxy is now being marketed to treat Rosacea and is costing sometimes $300+ for a month's supply. Malaria tx with Doxy. requires a 30-day supply plus the number of days you will be in the malarial zone. I usually look online for malarial zones like Siem Reap or Sihanoukville b/c the docs. will check to see if malaria is prevalent there before giving the RX. Doxy is Thailand is much cheaper than Doxy in the US. I do not know if this holds true in other Western lands.
Many pharmacists are wannabe physicians and playing to their desire to be a medical care provider (which to a large degree they already are) will get you much farther than simply walking in and asking for a certain med. Some pharmacists may provide what you want and simply not care but I have gotten much more by describing symptoms and letting the pharmacist do the diagnosis. A Z-pak on every trip is not a bad idea given that Chlamydia is often asymptomatic in men and 1 gram (1000 mgs) is the dose needed to knock it out. Doxy will knock out many other things but obviously one should see a doctor back home for a complete STD screening to be on the safe side. I do this after every trip.