Unless it's an opiate, an abused drug, or requires a doctor to monitor for side effects it's gennerally available OTC, but small pharmacies may not stock all drugs. Across the river from Thammasat University, over in Bangkok Noi there are lots of full service pharmacies near Siriraj Hospital and Wang Lang pier,. Check out Wang Lang market while you're there. It's a cool local market.
both places have excellent food courts. I spent over a year in BKK before moving to the provinces. Great city. I like staying out further from central, Udom Suk area is nice.
going through the Thai embassy in your home country for a "retirement visa" has it's advantages. They tend to stick to Bangkoks rules, and income statements are easier to provide.
I've done 9 border runs, 3 turn arounds and never had an issue. Obtain a 6 month TR60 multiple entry while out of the country. $200 usd 6 month validity 60 day exit re-entry required, or 30 day extension.
I'm currently in Thailand on a 6 month multiple entry visa. I've used it on every visit. I answered 6 months. It's a TR60 as stamped in your passport for US passport holders.