One of the more common antiepileptic medications is also one of the oldest – phenytoin. The medication reduces the risk for seizures by decreasing the activity of neuronal sodium channels.
Benefits of phenytoin include the ability of IV load and easy to measure drug levels which correlate well with clinical efficacy. Phenytoin is highly protein bound, and we can measure both the total and free (unbound) components of the drug. The free level is the therapeutic level, but is more difficult to test for in the lab.
For most patients, the goal therapeutic dose is a free level between 1.5 – 2.0. In most cases, there is a 10:1 ratio between the free and total component of the drug, correlating to a total phenytoin level of 15-20.
Drawbacks of phenytoin include the possibility of thrombophlebytis during IV infusion and bradycardia. These risks can be minimized by using fosphenytoin for IV loading. Long term use (over many years) can lead to gingival hyperplasia and ataxia. Phenytoin toxicity can cause nystagmus and ataxia.
Overall, phenytoin has lots of advantages such as easy IV loading and measurable levels, making it useful in the hospital. However, it is used less and less frequently as a long term, outpatient therapy.