This prevents them from multiplying, invading other red blood cells, and clogging arteries to our brain and other vital organs. As a result we don't suffer the fevers, chills, headaches and other joys of malaria. We continue our antimalarials for four weeks after leaving a malarious area in the hope that they will continue to mask, or suppress any symptoms until the risk of disease has likely passed. The drugs are worse than the disease. Tell that to someone who has almost died of malaria.
Antimalarials do have side effects in some people, some of the lime.
All drugs do. The side effects are usually transient and tolerable. Serious adverse reactions such as psychosis or seizures are rare. Compare that to the real thing, which may be fatal. I am immune to malaria.