Thailand's ability to deliver low cost, high quality medical care extends beyond hospital care. Thanks to government price controls, medicines can generally be purchased for far less than they can in most western countries. Most common prescription drugs are readily available, including those commonly used to treat high blood pressure, cholesterol and other chronic conditions.
Almost all drugs can be purchased at a pharmacy without a doctor's prescription. However, this does not mean that all drugs are what's called "over the counter" in the USA. In fact, all drugs, including even aspirin, must be purchase from a licensed pharmacy. There was time when all drug stores kept everything behind the counter, and you had to consult a pharmacist just to stock up on Tylenol. Since the introduction of western drug stores, such as Boots from the UK, things have relaxed a bit. Most medicines that would be considered over-the-counter items in other countries are now available from shelves around the pharmacist's counter. You'll still have to ask the special drugs that are considered prescription drugs in your home country.
While a pharmacist can dispense just about any medicine, certain drugs that are considered dangerous, such as morphine and its derivatives, require strict record-keeping on the part of the pharmacist. Most pharmacies don't want the hassle, so they don't carry so-called "class three" medicines.
One thing to make special note of is that while medicines are relatively cheap due to their classification as 'necessary' items, the same is not true of vitamins and other food supplements. These are not seen as essential, and thus can be relatively expensive. In fact, since many of them are imported, they may well be more expensive than back home.