We have seen many travellers, who have been on prescription
for malaria prevention, either Malarone or doxycycline. While travelling to Thailand,
they have unpleasant side effects, gastrointestinal issues and a few other
complaints and feel that we should provide some sound advice. 
This is the information from the CDC website regarding the
risk of malaria in Thailand. Chiang Mai, Bangkok and all the southern tourist
attraction islands are very, very low risk to practically no risk at all.
Your doctor back home might think that Thailand is a jungle
and would feel that it would be safer for you to take the malaria prevention pills,
but you can also find out for yourself that there is no huge risk here, there
is no need to make your body suffer by taking malaria medication. 

http://www.fitfortravel.nhs.uk/destinations/asia-(east)/thailand/thailand-malaria-map.aspx

(Malaria Area Map)

http://www.cdc.gov/malaria/travelers/country_table/t.html
(CDC Recommendation)