Primidone (Mysoline)

When is primidone used?

Primidone is used either alone or with other seizure medicines to control generalized tonic-clonic and partial seizures.

It is also used to treat myoclonic seizures and other seizure types in people with juvenile myoclonic epilepsy.

In what other conditions is primidone used?

Often doctors find that medicines are useful for more than one purpose. It is legal to prescribe medicines for "off-label uses" even though the U.S. Food and Drug Administration (FDA) has not formally approved such use. Besides epilepsy, one off-label use of primidone is the treatment of tremors—specifically, essential tremor (ET), the most common movement disorder.

What are the most common side effects of primidone?

The most common side effect of primidone, particularly with initial use, is fatigue. Side effects are generally mild and tend to decrease with time. Other side effects include:

  • unsteadiness
  • a spinning sensation (vertigo)
  • nausea
  • vomiting
  • blurred or double vision
  • drowsiness
  • depression

Less frequent side effects could include:

  • decreased appetite
  • irritability
  • acne or other skin lesions
  • sexual impotency
  • decreased sexual desire
  • blood abnormalities
  • frozen shoulder

If any of these symptoms persist, or are truly bothersome, call your doctor, but never stop or reduce the medicine on your own. Sometimes the doctor can help with these side effects by changing the prescription:

  • reducing the overall amount of primidone
  • changing the amount taken at certain times, such as taking a greater proportion of the primidone at bedtime to reduce daytime sleepiness
  • prescribing smaller doses, at more frequent intervals

You should never stop taking primidone, change the amount you take or when you take it without your doctor's guidance.

If you have just started taking primidone (or have just started taking a larger amount) you should be careful during activities that might be dangerous, until you know whether you are having any side effects.

In general, it is wise to avoid alcohol while taking primidone. Adults with a responsible approach to drinking (and who are taking a low to medium dose of primidone) may want to talk to the doctor about the possibility of having an occasional drink or two. Most doctors recommend that primidone should never be combined with alcohol, narcotics, tranquilizers, or antihistamines as the dangers of mixing primidone and any of these substances are severe and could include coma and death.

Since an active breakdown product of primidone in the body is phenobarbital, the same precautions for phenobarbital generally apply to primidone.

Can primidone be taken with other medicines?

As with phenobarbital, primidone can be taken with other medicines. However, since phenobarbital is an active break down product of primidone and since phenobarbital interferes with other medicines, primidone may also affect other medications. In particular, it may lower the level in the blood of several other seizure medicines including:

  • Carbamazepine (Tegretol, Carbatrol)
  • Lamotrigine (Lamictal)
  • Zonisamide (Zonegran)
  • Tiagabine (Gabitril)
  • Valproic acid (Depakote, Depakene)

Medicines often used for other disorders may also be affected. It is especially important to check with your doctor before taking any of these:

  • Antidepressants
  • Allergy medicines
  • Coumadin (warfarin)
  • Theophylline
  • Tagamet (cimetidine)
  • Oral contraceptives (birth control pills)
  • Narcotic pain relievers such as Percocet
  • Sedatives such as Nembutal and Seconal
  • Steroids such as Medrol and Deltasone
  • Tranquilizers such as Xanax and Valium

As with Phenobarbital, primidone can decrease the effectiveness of hormonal contraception including the birth control pill, hormonal injections or implants. Therefore women using these methods of birth control should be especially cautious as unplanned pregnancies are more likely to occur. To prevent pregnancy, a woman taking primidone may need to use a different type of birth control, raise the dosage of the contraceptive or increase the frequency of the implant or injection. She should tell both the doctor prescribing contraception and the doctor prescribing primidone about the other medicine so that the appropriate doses can be chosen. Primidone does not affect barrier types of birth control, like condoms, non-hormonal IUDs, and diaphragms.

What are the most serious side effects of primidone?

Most people who take primidone have no side effects or mild side effects that go away, but a very small number of people have serious reactions. Following is a list of symptoms that may be the start of a serious problem. If you notice any of these symptoms, call your doctor immediately:

  • An allergic reaction (difficulty breathing; closing of the throat; swelling of the lips, tongue, or face; or hives)
  • Fever, swollen glands, sore throat, or sores in the mouth (could mean a blood problem)
  • Red or purple point-like rash or blistering or peeling skin lesions
  • Easy bruising, paleness, excessive weakness or fatigue
  • Worsening of seizures

Who should not take primidone?

People who have had a condition called porphyria should not take primidone. Furthermore, those who are sensitive to phenobarbital are advised to avoid it, because the body produces phenobarbital when it processes primidone.

Women who are pregnant or may become pregnant, and those who are breast-feeding should be cautious about using primidone. The manufacturer suggests that breast-feeding be discontinued if the baby is unusually sleepy. Other seizure medicines may be a better choice for pregnant women and nursing mothers.

If a woman takes primidone during pregnancy,
will it hurt the baby?

Birth defects are probably a bit more common in the babies of women who take primidone during pregnancy than in others. A large majority of women who take primidone do have healthy, normal babies, however. The risk of defects is higher for women who take more than one seizure medicine and women with a family history of birth defects.

All women who are capable of becoming pregnant should take at least 0.4 mg (400 mcg) of the vitamin called folic acid every day because it helps to prevent specific birth defects called Neural Tube Defects. (The most well-known of these is spina bifida, in which the spinal cord is not completely enclosed.) Women with epilepsy should take between 1 and 4 mg of folic acid daily during their reproductive years. If the doctor thinks a woman is at especially high risk, the larger dose of folic acid—4 mg (4000 mcg) per day—may be recommended, beginning before the woman becomes pregnant.

Some babies born to mothers taking antiepileptic medications have had inadequate blood clotting within the first 24 hours after birth. It is often recommended that the mother be given 10 to 20 mg of vitamin K per day during the last month of pregnancy to prevent this problem. About 80% of primidone in the mother’s blood passes through breast milk, however how much of this actually enters the baby’s bloodstream is not known. Further, how primidone affects the baby is unknown, although the most common side effects are sedation and poor suck. That said, it is strongly felt by neurologists and epileptologists that the benefits of breastfeeding largely outweigh the risks of exposing the baby to antiepileptic medications, including primidone. As a matter of fact, the American Academy of Neurology and the American Epilepsy Society both recommend breastfeeding in women with epilepsy.

If you are a woman with epilepsy taking primidone and would like to become pregnant, you should discuss this with your doctor.