Verapamil with Fosphenytoin Interaction Details
Brand Names Associated with Verapamil
- Calan®
- Calan® SR
- Covera® HS
- Iproveratril Hydrochloride
- Isoptin®
- Tarka® (as a combination product containing trandolapril and verapamil)
- Verapamil
- Verelan®
- Verelan® PM
Brand Names Associated with Fosphenytoin
- Cerebyx®
- Fosphenytoin Injection
Medical Content Editor Dr. Brian Staiger, PharmD
Last updated
Dec 03, 2023
Interaction Effect
Decreased verapamil effectiveness
Interaction Summary
Concomitant use of some but not all calcium channel blockers (ie, verapamil, nimodipine, and felodipine) and phenytoin has been reported to decrease serum levels of the calcium channel blocker. Phenytoin-caused enzyme induction is probably responsible for this effect.
Severity
Moderate
Onset
Delayed
Evidence
Probable
How To Manage Interaction
Monitor for continuing clinical effectiveness of verapamil. Larger than expected doses may be required. Other calcium channel blockers may be tried cautiously since some of these agents have been shown to interact with phenytoin to produce symptomatic, toxic elevations in serum phenytoin levels. Use of an alternative therapeutic class of drug (ACE inhibitors, for example) may be required to minimize the possibility for a phenytoin interaction.
Mechanism Of Interaction
Increased verapamil metabolism
Literature Reports
A) Verapamil concomitantly administered with phenytoin causes a decrease in verapamil serum levels. A 28-year-old patient with suspected epilepsy was stabilized on phenytoin when verapamil was added to control symptoms caused by a left ventricular obstruction. Verapamil plasma levels were subtherapeutic, regardless of dose increases to 160 mg three times daily, until phenytoin was discontinued after one year without seizures. The verapamil plasma level significantly increased without additional doses. Phenytoin is a liver enzyme inducer, which appears to be the explanation of this interaction .
Verapamil Overview
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Verapamil is used to treat high blood pressure and to control angina (chest pain). The immediate-release tablets are also used alone or with other medications to prevent and treat irregular heartbeats. Verapamil is in a class of medications called calcium-channel blockers. It works by relaxing the blood vessels so the heart does not have to pump as hard. It also increases the supply of blood and oxygen to the heart and slows electrical activity in the heart to control the heart rate.
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High blood pressure is a common condition and when not treated, can cause damage to the brain, heart, blood vessels, kidneys and other parts of the body. Damage to these organs may cause heart disease, a heart attack, heart failure, stroke, kidney failure, loss of vision, and other problems. In addition to taking medication, making lifestyle changes will also help to control your blood pressure. These changes include eating a diet that is low in fat and salt, maintaining a healthy weight, exercising at least 30 minutes most days, not smoking, and using alcohol in moderation.
Fosphenytoin Overview
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Fosphenytoin injection is used to treat primary generalized tonic-clonic seizures (formerly known as a grand mal seizure; seizure that involves the entire body) and to treat and prevent seizures that may begin during or after surgery to the brain or nervous system. Fosphenytoin injection may also be used to control certain type of seizures in people who cannot take oral phenytoin. Fosphenytoin 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.