Take your personal prescribed medication as you may not find your preferred brands in Sri Lanka. Standalone pharmacies however are located in all cities and most major supermarkets have their own pharmacy counters within the store premises.
It is a good idea to take insect and mosquito repellent and sunscreen/suntan lotions with a high sun protection factor (SPF 50+) as these are surprisingly hard to come by in Sri Lanka. With so much sun to enjoy taking your UV repelling sun hats and sun glasses will prove useful.