A Cochrane review included 5 RCTs with a total of 439 subjects. Three trials compared high stimulation to low stimulation in participants aged 12-60 years and another trial made the same comparison in children. One trial compared 3 different treatments in patients over 12 years of age. Baseline phase ranged from 4 to 12 weeks and double-blind treatment phases from 12 to 20 weeks in the trials. Four trials compared high frequency stimulation to low frequency stimulation and were included in quantitative syntheses (meta-analyses).The overall risk ratio (RR) for 50% or greater reduction in seizure frequency across all studies was 1.73 (95% CI 1.13 to 2.64; 4 trials, n=373) showing that high frequency VNS was over one and a half times more effective than low frequency VNS. The RR for treatment withdrawal was 2.56 (0.51 to 12.71; 4 trials, n=375). The RR of adverse effects were as follows: (a) voice alteration and hoarseness 2.17 (99% CI 1.49 to 3.17); (b) cough 1.09 (99% CI 0.74 to 1.62); (c) dyspnea 2.45 (99% CI 1.07 to 5.60); (d) pain 1.01 (99% CI 0.60 to 1.68); (e) paresthesia 0.78 (99% CI 0.39 to 1.53); (f) nausea 0.89 (99% CI 0.42 to 1.90); (g) headache 0.90 (99% CI 0.48 to 1.69).