I was told by my doctor, that to take malaria meds for more than 30 days was hazardous to your kidneys and/or liver.
So best to wait until you get flu symptoms and then start taking the meds right away, and get to a hospital/doctor to be tested, if you've got it, then they meds they give for it will be the same, so you just keep taking it. If you don't have it, you don't risk much.
But the "preventative" doesn't actually prevent it, what it does is get the medicine in your blood stream early, so the symptoms are reduced. If you start taking the meds at the first sign of flu symptoms, you get about the same effect.
My wife, and her family, have all had numerous bouts of malaria, the first few times they take the meds, and then the next few times the symptoms are significantly less and they don't take the meds, by the 5th or 6th time, it's just a few days "flu".