When it was tested as an add-on treatment for epilepsy before its FDA approval, 17% of the people taking Vimpat 400 mg per day stopped taking it or lowered their dosage because of side effects, compared to 5% of people taking an inactive placebo.

The table below shows side effects of Vimpat 400 mg per day, versus side effects on placebo. The third numerical column is the difference.

Individual patients reported various side effects, though it usually was not clear that these resulted from Vimpat versus one of their concurrently taken seizures medicines or other causes. These included: heart racing or irregularity, ringing in the ears, constipation, stomach upset, dry or tingling mouth, irritability, feverishness, a drunken feeling, muscle spasms, skin tingling, thinking problems, slurred speech, disturbance in attention, imbalance, confusion, mood alteration, depression. One patient in 4011 who took Vimpat developed a hypersensitivity reaction with liver and kidney failure. This person recovered. Depression and increased risk of suicide have been noted with the general class of antiepileptic drugs. Additional side effects will become evident as more experience is gained with the drug. Certain infrequent side effects do not appear until hundreds or thousands have taken the drug under a variety of conditions or for many years.

Laboratory abnormalities

Blood tests sometimes show abnormalities in patients taking Vimpat. Liver abnormalities were seen in blood tests of 7 of 935 patients (0.7%). A few patients had lowering of blood counts which could potentially lead to anemia or lowering of resistance to infection. Electrocardiograms (EKGs) of heart rhythms showed prolongation of conduction of the electrical signal in the heart (called PR prolongation) in 0.4% (4/944) of patients randomized to receive VIMPAT and 0% (0/364) of patients randomized to receive placebo.