Are mefloquine side effects serious enough to stop it?

Sometimes. Mefloquine is used to prevent malaria. In some people, it can cause unpleasant side effects such as vivid dreams or anxiety or depression. In those with tendency towards seizures it can cause a seizure. All these effects happen because the drug can affect your brain. Note: many people tolerate the medication just fine. Talk to your doctor to see if it is right for you.