Valacyclovir with Phenytoin Interaction Details
Brand Names Associated with Valacyclovir
- Valacyclovir
- Valtrex®
Brand Names Associated with Phenytoin
- Dilantin®
- Phenytek®
- Phenytoin
Medical Content Editor Dr. Brian Staiger, PharmD
Last updated
Nov 17, 2023
Interaction Effect
Decreased phenytoin plasma concentrations and potential increased seizure activity
Interaction Summary
A case report documented a reduction in plasma levels of both phenytoin and valproic acid when combined with acyclovir treatment. This reduction resulted in significantly increased seizure activity from approximately 1 per month to 25 in one day.
Severity
Moderate
Onset
Delayed
Evidence
Probable
How To Manage Interaction
Monitor patients for reduction in phenytoin plasma levels. Consider alternative antiviral therapy.
Mechanism Of Interaction
Increased gastrointestinal transit or change in gastrointestinal fluid pH
Literature Reports
A) According to a case report from the University of Bologna in Italy, a 7-year-old boy with a history of epilepsy experienced increased seizure activity after being treated with acyclovir in addition to his antiepileptic medications. The patient's trough plasma levels of phenytoin and valproic acid were 17 mcg/mL and 32 mcg/mL, respectively, 10 days before starting acyclovir treatment for viral throat and mouth lesions. Four days after initiation of acyclovir treatment, the trough levels were 5 mcg/mL and 22 mcg/mL, respectively. Acyclovir treatment was discontinued after 6 days. Three and 6 days after acyclovir withdrawal, phenytoin and valproic acid plasma levels were still low, and the patient experienced 25 serial partial seizures 5 days after discontinuation. Phenytoin dosage was increased to reach therapeutic plasma levels and the frequency of seizures was reduced to 2 or 3 per week. The authors suggest that further study of this possible interaction is warranted .
Valacyclovir Overview
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Valacyclovir is used to treat herpes zoster (shingles) and genital herpes. It does not cure herpes infections but decreases pain and itching, helps sores to heal, and prevents new ones from forming.
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This medication is sometimes prescribed for other uses; ask your doctor or pharmacist for more information.
Phenytoin Overview
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Phenytoin is used to control certain type of seizures, and to treat and prevent seizures that may begin during or after surgery to the brain or nervous system. Phenytoin is in a class of medications called anticonvulsants. It works by decreasing abnormal electrical activity in the brain.
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Definitions
Severity Categories
Contraindicated
These drugs, generally, should not be used together simultaneously due to the high risk of severe adverse effects. Combining these medications can lead to dangerous health outcomes and should be strictly avoided unless otherwise instructed by your provider.
Major
This interaction could result in very serious and potentially life-threatening consequences. If you are taking this drug combination, it is very important to be under close medical supervision to minimize severe side effects and ensure your safety. It may be necessary to change a medication or dosage to prevent harm.
Moderate
This interaction has the potential to worsen your medical condition or alter the effectiveness of your treatment. It's important that you are monitored closely and you potentially may need to make adjustments in your treatment plan or drug dosage to maintain optimal health.
Minor
While this interaction is unlikely to cause significant problems, it could intensify side effects or reduce the effectiveness of one or both medications. Monitoring for changes in symptoms and your condition is recommended, and adjustments may be made if needed to manage any increased or more pronounced side effects.
Onset
Rapid: Onset of drug interaction typically occurs within 24 hours of co-administration.
Delayed: Onset of drug interaction typically occurs more than 24 hours after co-administration.
Evidence
Level of documentation of the interaction.
Established: The interaction is documented and substantiated in peer-reviewed medical literature.
Theoretical: This interaction is not fully supported by current medical evidence or well-documented sources, but it is based on known drug mechanisms, drug effects, and other relevant information.
How To Manage The Interaction
Provides a detailed discussion on how patients and clinicians can approach the identified drug interaction as well as offers guidance on what to expect and strategies to potentially mitigate the effects of the interaction. This may include recommendations on adjusting medication dosages, altering the timing of drug administration, or closely monitoring for specific symptoms.
It's important to note that all medical situations are unique, and management approaches should be tailored to individual circumstances. Patients should always consult their healthcare provider for personalized advice and guidance on managing drug interactions effectively.
Mechanism Of Interaction
The theorized or clinically determined reason (i.e., mechanism) why the drug-drug interaction occurs.
Disclaimer: The information provided on this page is for informational purposes only and should not be considered medical advice. Always consult with a qualified healthcare professional regarding your specific circumstances and medical conditions.
Where Does Our Information Come From?
Information for our drug interactions is compiled from several drug compendia, including:
The prescribing information for each drug, as published on DailyMED, is also used.
Individual drug-drug interaction detail pages contain references specific to that interaction. You can click on the reference number within brackets '[]' to see what reference was utilized.
The information posted is fact-checked by HelloPharmacist clinicians and reviewed quarterly.