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Phenytoin (Dilantin) Mass Concentration in Serum

Published: August 9, 2026Updated: August 16, 2026

This test measures the amount of phenytoin, a medication used to control certain types of seizures, in your blood. It helps ensure your dose is at a safe and effective level.

Quick summary

["Phenytoin levels help prevent toxicity while maintaining seizure control."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
105963-3
Short code
Zolpidem phenyl-4-carb Ur-mCnc
Synonyms
Zolpidem phenyl-4-carb Ur-mCnc
Category
chemistry
Sample
urine
Container
Sterile urine container
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Specimen volume
10 mL
Stability
Room temperature: 2h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
0
Lipemia max
0
Icterus max
0
TAT
4–24 hours
Order
routine

Why This Test Is Done

Your healthcare provider may order this test if you’re taking phenytoin to manage epilepsy or other seizure disorders. It helps monitor your medication levels to avoid side effects like dizziness, nausea, or coordination problems while ensuring the drug works properly.

What to Expect Before the Test

No special preparation is needed. A small needle will be used to draw blood from a vein in your arm. You may feel a brief pinch, but it usually doesn’t cause discomfort afterward.

What the Test Results Mean

Results are used to guide your medication dose. If levels are too low, your seizures may not be controlled. If levels are too high, you may experience unwanted side effects. Your provider will discuss these findings with you.

Common Side Effects to Watch For

Phenytoin can cause side effects like dizziness, rash, or nausea. If you experience severe symptoms (e.g., trouble breathing, swelling, or irregular heartbeat), seek medical attention immediately.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Maintaining appropriate phenytoin levels is critical for seizure control and reducing risks of overdose or underdose. Blood levels help balance these risks, especially in people with kidney issues or those taking other medications that interact with phenytoin.

Short name

Phenytoin Level

Frequently asked questions

How often will I need this test?
Your provider will determine the frequency based on your treatment plan, seizure activity, and how your body responds to the medication. Regular monitoring may be needed if you’re on a stable dose or if you experience changes in symptoms.
What happens if my phenytoin level is too high or too low?
Too high: Symptoms may include confusion, rapid or irregular heartbeat, or severe dizziness. Too low: You might have increased seizure activity or difficulty controlling seizures. Your provider will adjust your dose accordingly.
Can I drink alcohol while taking phenytoin?
No, alcohol can increase the risk of side effects like dizziness or liver problems. Always follow your provider’s instructions for medication use and lifestyle adjustments.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy