If you can get it and are happy to pay a bit more than other options then I recommend Malarone. Much shorter course and most unlikely to cause side effects. That way you are likely to finish the course which is important.

I didnt have any worries with Malarone but maybe it cost me £60/70 or so, better than psychosis and hives though. Larium was mad.
Will I need them for Vietnam? maybe not if I stick to Nha Trang and Saigin? maybe I do for higher regions such as Dalat?

mmm..Malarone is quite expensive... but then I guess hospitalisation for malaria wouldnt be cheap either!

As a woman, I have to say that Doxycycline will make you very (and I mean VERY) susceptible to stubborn vaginal yeast infections. I was also constipated for weeks after coming back from SEA, but that could have had other reasons. Can't comment on the sun-sensitivity, as I burn very easily anyway.
I wish I could have the "use-Deet-and-hope-for-the-best attitude", but I have seen first hand what malaria can do to a strong, fit and formerly healthy man. So I will use Malarone.
(Besides being not recommended for Cambodia these days, Lariam caused some interesting psychotic side effects: I was in Vietnam and suddenly remembered that I had to send my sister a postcard. So I started to write one, and I couldn't remember her address. And then I remembered I didn't have a sister. That was Lariam. But the dreams were great. No nightmares, just extremely vivid.)
Just quick comment to dispell an old wives' tale that you don't need to worry abour malaria in major cities. It is just that, myth. Mosquitos love urban environments where humans kindly provide standing water. Now in my area of Secunderabad they put oil on the nearby lake for a very good reason. Wish they had always done so..

14# I think the chances of getting Malaria in Siem Reap and Phnom Phnom are very low. I know number of expats who have lived in Siem Reap and in Phnom Penh for a number of years and I have heard only 1 person get Malaria and that was probably because he went out into the outbacks for a few weeks.
Hi Vozzer,
I did the central highlands (Kontum and Playku) and Dalat in 2005 and was told by the 2 guides I had not to bother. I didn't and all was o.k. but of course that was only me !! Did see a lot of mozzies in Kontum but with the usual precautions like DEET spray, long sleeves etc. didn't get bitten. But once again perhaps my blood is too old for 'em.
Good luck