It's pretty unusual to take anything to go to Iquitos unless you are going out to a jungle retreat. The entire population of Iquitos seems happy enough without popping malaria pills. There's a steady population growth and the streets are not littered with unclaimed dead.
Malaria pills also have unpleasant side affects. And all vaccines contain horrendous amounts of mercury, conspiracy theories or not. They are generally to be avoided if at all possible... you have to strike a risk balance.
The best malaria prevention in existence are long sleeved t-shirts or light shirts. You will be sweating whatever you wear, even if you wear nothing, so what's the difference?