[3] Common side effects include nausea, stomach pain, loss of appetite, poor coordination, increased hair growth, and enlargement of the gums.

[3] Potentially serious side effects include sleepiness, self harm, liver problems, bone marrow suppression, low blood pressure, toxic epidermal necrolysis,[3] and atrophy of the cerebellum.

[17] Common side effects include nausea, stomach pain, loss of appetite, poor coordination, increased hair growth, and enlargement of the gums.

Potentially serious side effects include sleepiness, self harm, liver problems, bone marrow suppression, low blood pressure, and toxic epidermal necrolysis.

[3] Severe low blood pressure and abnormal heart rhythms can be seen with rapid infusion of IV phenytoin.

At toxic doses, patients experience vertical nystagmus, double vision, sedation, slurred speech, cerebellar ataxia, and tremor.

[24] Folate is present in food in a polyglutamate form, which is then converted into monoglutamates by intestinal conjugase to be absorbed by the jejunum.

[30][31] Data now being collected by the Epilepsy and Antiepileptic Drug Pregnancy Registry may one day answer this question definitively.

[36] Phenytoin has been associated with drug-induced gingival enlargement (overgrowth of the gums), probably due to above-mentioned folate deficiency; indeed, evidence from a randomized controlled trial suggests that folic acid supplementation can prevent gingival enlargement in children who take phenytoin.

Phenytoin therapy has been linked to the life-threatening skin reactions Stevens–Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN).

[38] This allele occurs almost exclusively in patients with ancestry across broad areas of Asia, including South Asian Indians.

[39] The U.S. Food and Drug Administration (FDA) notes on the phenytoin drug label that since strong evidence exists linking HLA-B*1502 with the risk of developing SJS or TEN in patients taking carbamazepine, consideration should be given to avoiding phenytoin as an alternative to carbamazepine in patients carrying this allele.

Vitamin D deficiency, as well as low calcium and phosphate in the blood cause decreased bone mineral density.

Antacids administered in a peptic ulcer regimen may decrease the AUC of a single dose of phenytoin.

Phenytoin reduces the maximal activity of brain stem centers responsible for the tonic phase of generalized tonic-clonic seizures.

In 1938, other physicians, including H. Houston Merritt and Tracy Putnam, discovered phenytoin's usefulness for controlling seizures, without the sedative effects associated with phenobarbital.

[54] According to Goodman and Gilman's Pharmacological Basis of Therapeutics: In contrast to the earlier accidental discovery of the antiseizure properties of potassium bromide and phenobarbital, phenytoin was the product of a search among nonsedative structural relatives of phenobarbital for agents capable of suppressing electroshock convulsions in laboratory animals.

He has claimed to have supplied large amounts of the drug to Richard Nixon throughout the late 1960s and early 1970s, although this is disputed by former White House aides[56] and Presidential historians.

[58] Despite more than $70 million in personal financing, his push to see phenytoin evaluated for alternative uses has had little lasting effect on the medical community.

[citation needed] In 2008, the drug was put on the FDA's Potential Signals of Serious Risks List to be further evaluated for approval.

To address this concern, the Warnings and Precautions section of the labeling for Dilantin injection was updated to include additional information about Purple glove syndrome in November 2011.

[61] After Pfizer's sale of the UK marketing licence to Flynn Pharma, the price of a 28-pack of 25 mg phenytoin sodium capsules marked Epanutin rose from 66p (about $0.88) to £15.74 (about $25.06).