In pregnant women, mefloquine appears to pose minimal risk to the fetus, and is not associated with increased risk of birth defects or miscarriages. Compared to other malaria chemoprophylaxis regimens, however, mefloqinone may produce more side effects in non-pregnant travelers. Mefloquine is also safe and effective for use during breastfeeding, though it appears in breast milk in low concentrations. : 9  The World Health Organization (WHO) gives approval for the use of mefloquine in the second and third trimesters of pregnancy and use in the first trimester does not mandate termination of pregnancy. Pharmacology
Elimination
Mefloquine is metabolized primarily through the liver. Its elimination in persons with impaired liver function may be prolonged, resulting in higher plasma levels and an increased risk of adverse reactions. The mean elimination plasma half-life of mefloquine is between two and four weeks. Total clearance is through the liver, and the primary means of excretion is through the bile and feces, as opposed to only 4% to 9% excreted through the urine. During long-term use, the plasma half-life remains unchanged.Liver function tests should be performed during long-term administration of mefloquine. Alcohol use should be avoided during treatment with mefloquine. Chemistry
Specifically it is used as mefloquine hydrochloride. Mefloquine is a chiral molecule with two asymmetric carbon centres, which means it has four different stereoisomers. The drug is currently manufactured and sold as a racemate of the (R,S)- and (S,R)-enantiomers by Hoffman-LaRoche, a Swiss pharmaceutical company. Essentially, it is two drugs in one.
Neurologic and psychiatric
In 2013, the U.S. Food and Drug Administration (FDA) added a boxed warning to the prescription label of mefloquine regarding the potential for neuropsychiatric side effects that may persist even after discontinuing administration of the medication. In 2013 the FDA stated "Neurologic side effects can occur at any time during drug use, and can last for months to years after the drug is stopped or can be permanent." Neurologic effects include dizziness, loss of balance, seizures, and tinnitus. Psychiatric effects include nightmares, visual hallucinations, auditory hallucinations, anxiety, depression, unusual behavior, and suicidal ideations. Central nervous system events requiring hospitalization occur in about one in 10,000 people taking mefloquine for malaria prevention, with milder events (e.g., dizziness, headache, insomnia, and vivid dreams) in up to 25%. When some measure of subjective severity is applied to the rating of adverse events, about 11–17% of travelers are incapacitated to some degree. Cardiac
Mefloquine may cause abnormalities with heart rhythms that are visible on electrocardiograms. Combining mefloquine with other drugs that cause similar effects, such as quinine or quinidine, can increase these effects. Combining mefloquine with halofantrine can cause significant increases in QTc intervals. : 10

Contraindications
Mefloquine is contraindicated in those with a previous history of seizures or a recent history of psychiatric disorders. Pregnancy and breastfeeding
Available data suggests that mefloquine is safe and effective for use by pregnant women during all trimesters of pregnancy, and it is widely used for this indication.