If you visit PubMed (http://www.ncbi.nlm.nih.gov/pubmed/), a service of the National Library of Medicine, you can read summaries of studies that have tested phenytoin, as well as other reports about this medication.

Here are links to some articles relevant to this subject:

Mattson, RH, Cramer, JA, et al. Comparison of carbamazepine, phenobarbital, phenytoin and primidone in partial and secondarily generalized tonic clonic seizures. N Engl J Med 313:145-151, 1985. PMID: 3925335.

Overall, phenytoin and carbamazepine were shown to be the first choices for a single medication to treat adults with newly diagnosed partial seizures, generalized tonic-clonic seizures, or both.

Heller AJ, Chesterman P, Elwes RD, et al. Phenobarbitone, phenytoin, carbamazepine, or sodium valproate for newly diagnosed adult epilepsy: a randomised comparative monotherapy trial. J Neurol Neurosurg Psychiatry 1995 Jan;58(1):44-50. PMID: 7823066.

Four medications were similar in their ability to control seizures, but phenytoin had the lowest rate of intolerable side effects.

Wilder BJ, Ramsay RE, Murphy JV, Karas BJ, Marquardt K, Hammond EJ. Comparison of valproic acid and phenytoin in newly diagnosed tonic-clonic seizures. Neurology 1983 Nov;33(11):1474-6. PMID: 6415511.

Valproic acid was slightly more effective than phenytoin in controlling generalized tonic-clonic seizures.

Read more about how to find and understand information in PubMed.