Interestingly, mefloquine and related quinoline anti-malarials have recently begun to be explored as treatments for certain types of central nervous system cancers. This makes sense, because these drugs readily penetrate the blood-brain barrier. They readily concentrate—sometimes at very high concentrations—in the brain, and they're neurotoxic; they kill brain cells. If brain cells are rapidly multiplying, as they do in cancer, drugs like mefloquine can have some theoretic benefit to treating those cancers.
The same property that renders these drugs inherently dangerous, in my opinion, when given to healthy service members, may make them very effective cancer agents.