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Phenytoin is an anticonvulsant that works by stabilizing the neuronal membrane and preventing the spread of seizure activity¹. The therapeutic range of phenytoin is **10-20 mcg/mL**²³⁴⁵. A serum phenytoin level of **25 mcg/mL** is above the therapeutic range and indicates toxicity. The nurse should hold the next dose and notify the provider to prevent further adverse effects, such as nystagmus, ataxia, confusion, and coma¹.

Phenytoin is an anticonvulsant that works by stabilizing the neuronal membrane and preventing the spread of seizure activity¹. The therapeutic range of phenytoin is **10-20 mcg/mL**²³⁴⁵. A serum phenytoin level of **25 mcg/mL** is above the therapeutic range and indicates toxicity. The nurse should hold the next dose and notify the provider to prevent further adverse effects, such as nystagmus, ataxia, confusion, and coma¹.

Phenytoin is an anticonvulsant that works by stabilizing the neuronal membrane and preventing the spread of seizure activity¹. The therapeutic range of phenytoin is **10-20 mcg/mL**²³⁴⁵. A serum phenytoin level of **25 mcg/mL** is above the therapeutic range and indicates toxicity. The nurse should hold the next dose and notify the provider to prevent further adverse effects, such as nystagmus, ataxia, confusion, and coma¹.

Choice A is wrong because valproic acid does not inhibit the breakdown of a neurotransmitter, but rather increases its availability. The neurotransmitter that is inhibited by valproic acid is not known to prevent seizures, but rather to cause them. This neurotransmitter is glutamate, which is the main excitatory neurotransmitter in the brain³.

Choice C is wrong because valproic acid does not block sodium channels that trigger seizures, but rather calcium channels that are involved in neuronal signaling. Blocking sodium channels is the mechanism of action of another class of anticonvulsants, such as phenytoin and carbamazepine⁴.

Choice D is wrong because valproic acid does not suppress calcium influx that causes seizures, but rather inhibits calcium channels that regulate calcium influx. Suppressing calcium influx is the mechanism of action of another anticonvulsant, ethosuximide, which is also used to treat absence seizures⁵.

Choice B is wrong because these are not signs of toxicity, but signs of dependence. Toxicity occurs when the drug reaches a level in the body that causes harmful effects, such as respiratory depression, hypotension, coma, or death⁵. Signs of toxicity may include slow or shallow breathing, weak pulse, cold or clammy skin, little or no urination, pinpoint pupils, feeling cold, or fainting².

Choice C is wrong because these are not symptoms of withdrawal, but signs of dependence. Withdrawal occurs when the drug is stopped or reduced after a period of regular use, causing the body and the mind to react to the absence of the drug. Withdrawal symptoms may include anxiety, restlessness, irritability, agitation, confusion, hallucinations, seizures, or delirium⁹. Increasing the dose of phenobarbital can worsen the dependence and increase the risk of adverse effects.

Phenobarbital is a barbiturate that can cause physical and psychological dependence with chronic use, meaning that the body and the mind become accustomed to the drug and need it to function normally. Dependence can lead to withdrawal symptoms when the drug is stopped or reduced, such as insomnia, nausea, tremors, and dizziness⁹. Dependence can also increase the risk of overdose, tolerance, and addiction¹³.