Phenobarbital - oral

Patient Medicine Information
Why do I need this medicine?
Phenobarbital is used to treat epilepsy (also known as fits or seizures).

This medicine may be used to treat other conditions as decided by your doctor.
How do I take this medicine?
Take Phenobarbital exactly as directed by your doctor or according to the instructions on the label. Do not take more or less than instructed by your doctor.

You may take this medicine with or without food. Try to take it at the same time each day.

Phenobarbital is available as a tablet or oral solution.

If you are taking the tablet, swallow it whole with a glass of water.

If you are taking the oral solution, shake the bottle well before you take it to ensure that the liquid is evenly mixed. Use the measuring spoon or cup provided to measure your dose.

The dose of this medicine will be decided by your doctor. Your doctor will advise you on the treatment timeframe depending on the type and severity of your condition and your response to the medication.

Phenobarbital must be taken regularly for it to be effective. Continue taking this medicine even when you feel better. Do not stop taking it suddenly as this may worsen your condition.
What should I do if I have forgotten to take this medicine?
Take the missed dose as soon as you remember. If it is almost time for your next dose, skip the missed dose and return to your normal dosing schedule.

DO NOT double a dose under any circumstances.

If you often forget to take your medicine, let your doctor and pharmacist know.
When should I not use this medicine?
Alert your doctor if you have the following conditions:
  • existing or history of acute intermittent porphyria (an inherited disorder that causes skin or nervous system abnormalities)
  • severe breathing difficulties
  • severe liver disease
  • severe kidney disease
as Phenobarbital may not be suitable for you.
What should I take note of while taking this medicine?
Inform your doctor if you have the following conditions:
  • lung disease e.g. severe asthma
  • long-term or severe pain
  • history of drug or alcohol abuse
  • severe anaemia (low red blood cell count in the body) or history of blood disease
  • heart disease
  • low blood pressure
  • depression or having thoughts of self-harm
  • diabetes
  • hyperthyroidism (overactive thyroid gland)
  • hypoadrenalism (a disorder where the adrenal glands produce insufficient hormones)
  • fever
  • mild to moderate kidney disease
  • mild to moderate liver disease
Let your doctor know if you are pregnant or breastfeeding.

Be sure to check with your doctor or pharmacist before giving Phenobarbital to a child, the elderly, or debilitated individual (physically or mentally weak person, usually due to illness or old age). These people may be more sensitive to the side effects.

You may need to take calcium and vitamin D supplements while being treated with this medicine. Your doctor will advise you when you need these supplements and how much to take.

If you are going for certain laboratory tests (e.g. metyrapone test, phentolamine test, blood bilirubin test), inform your doctor that you are taking this medicine.

Why is it important to keep my appointments with the doctor?

Keep your appointments with your doctor. Your doctor needs to monitor your condition and check your response to the medication regularly.
  • Routine tests (e.g. liver or kidney function, complete blood count) may be done while you are being treated with this medicine. Your doctor will advise you about how often you need to have these tests.
  • Regular monitoring of signs of depression or unusual changes in behaviour may also be needed.
What side effects could I experience?
Phenobarbital may cause drowsiness or make you feel less alert than usual. If affected, do not drive or take part in any activity in which you need to be alert.

Other side effects include any of the following: nausea, vomiting, lack of energy, difficulty in controlling or coordinating movements, memory problems, joint pain, and rashes.

Some side effects may need immediate medical help. Alert your doctor quickly if you experience any of the following:
  • rashes with peeling of the skin or blistering of the lips, mouth, or eyes accompanied by fever
  • difficulty breathing
  • unusual changes in mood or behaviour (being too depressed, agitated, or having thoughts of self-harm), hearing or seeing things that are not there, confusion
  • unusual bleeding or bruising
  • pale skin, swollen tongue, loss of appetite or weight loss
  • bone pain
Inform your doctor if any of these side effects do not go away or are severe, or if you experience other side effects.
Can I take this with other medicines?
Inform your doctor and pharmacist if you are taking any of these medicines:
  • medicines to treat depression e.g. isocarboxazid, paroxetine, nortriptyline, mianserin
  • other medicines for epilepsy (fits or seizures) e.g. oxcarbazepine, lamotrigine, phenytoin, vigabatrin
  • medicines for mood disorders e.g. chlorpromazine, aripiprazole, haloperidol
  • medicines for high blood pressure e.g. metoprolol, verapamil
  • medicines for irregular heartbeat e.g. digoxin, quinidine
  • medicines used in organ transplant or certain immune disorders e.g. ciclosporin, tacrolimus
  • certain antibiotics e.g. chloramphenicol, doxycycline, metronidazole
  • medicines to treat fungal infections e.g. itraconazole, voriconazole, griseofulvin
  • medicines for HIV infection e.g. indinavir, nelfinavir, saquinavir
  • certain medicines for cancer e.g. etoposide, irinotecan, toremifene
  • certain asthma medicines e.g. montelukast, theophylline
  • medicines for thyroid problems e.g. levothyroxine
  • blood-thinning medicines e.g. warfarin, dicoumarol
  • corticosteroids (anti-inflammatory medicines) e.g. prednisolone
  • aprepitant (medicine for nausea or vomiting caused by cancer medicines)
  • rifampicin (medicine to treat TB, a lung infection known as tuberculosis)
  • methylphenidate (medicine to treat hyperactivity disorders)
  • methadone (strong painkiller)
  • paracetamol (medicine used to relieve fever or pain)
  • other medicines that can cause drowsiness or affect your concentration
  • vitamin D supplements
  • folic acid supplements
  • St. John's wort (herbal medicine)
This list does not include all medicines that may interact with Phenobarbital.

Inform your doctor if you are currently taking birth control pills as Phenobarbital may reduce the effectiveness of hormonal contraceptives. You should not stop or start any birth control pills without first informing your doctor. Ask your doctor about using non-hormonal birth control as an alternative while being treated with this medicine.

Always notify your doctor and pharmacist if you are taking any other medicines, including herbal tonics such as traditional Chinese medicines, supplements and medicines that you buy without a prescription.
What special dietary instructions should I follow?
Avoid alcohol. Alcohol intake may worsen the drowsiness caused by Phenobarbital.
How should I store this medicine?
Store in a cool, dry place away from the reach of children.

Protect from light.

Medicines must not be used past the expiry date.
This information is independently developed by MIMS based on phenobarbital - oral and is provided for your reference only. It is not a replacement for and should only be used in conjunction with full consultation with a licensed healthcare professional, the information provided by your pharmacist and/or the manufacturer of the medication. It may not contain all the available information you require and cannot substitute professional medical care, nor does it take into account all individual circumstances. Although great effort has been made to ensure content accuracy, we shall not be held responsible or liable for any claims or damages arising from the use or misuse of the information contained herein, its contents or omissions, or otherwise. Copyright © 2022 MIMS. All rights reserved. Powered by
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