The first thing we should say about malaria is that we are not doctors! Check with your GP for professional advice.
Speaking from personal experience in the region, we didn’t take anti-malarials and we haven’t had any problems. We have tried to minimise the risk of being bitten by applying mosquito repellent and covering up our skin in the early morning and evening. We also made sure that all hotels we stayed in were either well sealed or had mosquito nets.
We didn’t find mosquitos to be a major problem anywhere.
From our research, we believe the risk of malaria in Thailand to be almost non-existant, except along the Cambodian border. In Cambodia, the heavily forested northern and western areas of the country do present a malaria risk but there is no risk in the main cities like Siem Reap and Phnom Penh.
The reason we didn’t take anti-malarials was because of the risk of side effects but again, you should make your decision after talking to a doctor. Malaria is a serious disease if you catch it so it’s worth taking the time to research.
Perhaps a bigger risk for travellers is dengue fever, also transmitted by mosquitos and yet another reason to cover up and avoid being bitten in the first place.