Thailand's pharmacy system is excellent — what's available over the counter and which chains to use.
Antibiotics (most types), antifungals, antiparasitics, antihistamines, NSAIDs, antidiarrhoeals, motion sickness, sleeping aids — significantly more than in Western countries.
Boots (most familiar to British visitors), Fascino, and local Thai pharmacy chains throughout Bangkok; 7-Eleven stocks basic OTC medications too.
Thai pharmacists are often excellent and take time to help — describe symptoms and they'll often diagnose and recommend; English is usually sufficient in tourist areas.
Bring a letter from your doctor for controlled medications (benzodiazepines, opiates, ADD medication); some are strictly regulated despite being legal.
Medications are significantly cheaper than in Western countries — a course of antibiotics that would cost $50+ in the US is typically 150–400 THB in Bangkok.
