phenytoin sodium
Meaning of phenytoin sodium: A long-serving seizure medicine with an unforgiving dose response, where a small increase can push blood levels far higher than expected.
Definition of phenytoin sodium
Phenytoin sodium is the salt form of phenytoin, a medicine that prevents and stops seizures by slowing the recovery of sodium channels in nerve cells, which prevents them firing in rapid runs. It is used for focal seizures, for generalized convulsive seizures, for seizure activity that will not stop, and around brain surgery or head injury. The brand name Dilantin is widely recognized.
Why is the dosing so delicate?
Because the liver enzymes that clear the drug can be saturated. At low levels the body removes a fixed proportion of whatever is present, which is how most medicines behave and which makes the effect of a dose change easy to predict. Once those enzymes are working flat out, only a fixed amount can be removed per hour no matter how much is there. A modest increase in dose beyond that point can send blood levels far higher than intended, and a small reduction can drop them steeply. That is why measuring the blood level is routine with this drug and uncommon with most others, and why dose and level do not move in step.
What does long-term use look like?
| Effect | What is seen |
|---|---|
| Gum overgrowth | Gums thicken and creep over the teeth |
| Hair and facial change | Coarser features and extra body hair |
| Bone thinning | Through altered handling of vitamin D |
| Balance and eye movement | Unsteadiness and flickering eye movements as levels climb |
The last of these is the earliest visible sign that a level has risen too far, followed by slurred speech and confusion. The drug also switches on liver enzymes strongly, lowering the levels of many medicines taken alongside it, hormonal contraception among them.
Which seizures does it not help?
It is not effective against absence seizures, the brief blank episodes of childhood, and it can make those and myoclonic jerks worse, so establishing the seizure type comes before choosing it. It is an anticonvulsant of long standing, sold as Dilantin among other names, and it carries a recognized risk of harm to a developing baby along with severe skin reactions that occur more often in people carrying a particular tissue type.
Also known as
Used in a sentence
Phenytoin sodium was loaded intravenously for status epilepticus.
Good to know
Clearance of phenytoin saturates, so blood levels rise disproportionately once a threshold dose is passed, and the drug is ineffective in absence and myoclonic seizures and can aggravate them.