According to Goodman and Gilman's Pharmacological Basis of Therapeutics,
There are some indications that phenytoin has other effects, including anxiety control and mood stabilization, although it has never been approved for those purposes by the FDA. Jack Dreyfus, founder of the Dreyfus Fund, became a major proponent of phenytoin as a means to control nervousness and depression when he received a prescription for Dilantin in 1966. Notably, he is believed to have supplied large amounts of the drug to Richard Nixon throughout the late 1960s and early 1970s. Dreyfus' book about his experience with phenytoin, A Remarkable Medicine Has Been Overlooked, sits on the shelves of many physicians courtesy of the work of his foundation. 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. This was partially because Parke-Davis was reluctant to invest in a drug nearing the end of its patent life, and partially due to mixed results from various studies.
On September 5, 2008 the drug was put on the FDA's Potential Signals of Serious Risks List to be further evaluated for approval. The list means that the FDA has identified a potential safety issue, but does not mean that FDA has identified a causal relationship between the drug and the listed risk.
According to the FDA's New Safety Information Identified by the Adverse Event Reporting System (AERS) Phenytoin Injection (Dilantin) has been associated with the risk of Purple Glove Syndrome.
It has been suggested that phenytoin causes a reduction in folic acid levels, predisposing patients to megaloblastic anemia. Folic acid is presented as polyglutamate in foods, it is then converted into monoglutamates by intestinal conjugase. Phenytoin acts by inhibiting this enzyme therefore causing folate deficiency.
Phenytoin is a known teratogen. The syndrome consists of craniofacial anomalies (broad nasal bridge, cleft lip and palate, microcephaly) and a mild form of mental retardation (average IQ=71). This syndrome resembles the well-described Fetal Alcohol Syndrome and has also been called the "fetal hydantoin syndrome."
Recently, the teratogenicity of phenytoin has been contested. One blinded trial asked physicians to separate photographs of children into two piles based on whether they showed the so-called characteristic features of this syndrome; it found that physicians were no better at diagnosing the syndrome than would be expected by random chance, calling the very existence of the syndrome into question. Data now being collected by the Epilepsy and Antiepileptic Drug Pregnancy Registry may one day answer this question definitively.
Phenytoin may accumulate in the cerebral cortex over long periods of time, as well as causing atrophy of the cerebellum when administered at chronically high levels. Despite this, the drug has a long history of safe use, making it one of the more popular anti-convulsants prescribed by doctors, and a common "first line of defense" in seizure cases.
Recently phenytoin is suggested to be a human carcinogen.
Due to patent expiration, phenytoin is available in generic form and several branded forms at relatively low cost, making it one of the more affordable seizure control medications. It is available in extended release capsules and injectable forms, though the injectable formulation is rapidly losing ground to fosphenytoin (an important side note is that fosphenytoin has to be dephosphorylated before it can metabolized for use which can take an extra 15 minutes). Some generic formulations of phenytoin have been felt to be less reliable with respect to time-release than their branded counterparts. In some cases, this can be related to complications which arise between the alternative protein bond release mechanisms used in the generic versions, and those individuals with high metabolic rates.
Phenytoin has been associated with drug induced gingival enlargement (hyperplasia) in the oral cavity probably due to above mentioned folate defiency. Plasma concentrations needed to induce gingival lesions has not been clearly defined. Effects consist of the following: bleeding upon probing, increased gingival exudate, pronounced gingival inflammatory response to plaque levels, associated in some instances with bone loss but without tooth detachment.
Following almost 200 studies of 11 anti-seizure drugs, the FDA has also warned of an increased suicide risk for any patients treated with certain anti-seizure drugs. The study of 44,000 patients found that patients whose epilepsy is treated with drugs face about twice the risk of suicidal thoughts compared to placebo-takers. Although phenytoin was not named in the study, the FDA announced that it expected the risk applied to every epilepsy drug.