Primidone is an anticonvulsant medication used primarily to control seizures. Sold under the brand name Mysoline, it has been available since the 1950s and remains in use today for both epilepsy and, perhaps more commonly in current practice, essential tremor. If your doctor prescribed primidone or you saw it on a medication list, here’s what it does and what to expect.
Seizure Control
Primidone is FDA-approved to treat epileptic seizures. It works for several seizure types, including generalized tonic-clonic seizures (formerly called grand mal) and focal seizures that start in one area of the brain. It can be used alone or alongside other seizure medications.
What makes primidone unusual is that it doesn’t just work as one drug. Your liver breaks it down into two active compounds, one of which is phenobarbital, a well-known anticonvulsant on its own. So when you take primidone, you’re essentially getting the anti-seizure effects of three substances: primidone itself, phenobarbital, and a second breakdown product called PEMA. Each of these stays active in your body for different lengths of time. Primidone clears relatively quickly, with a half-life of about 4 to 12 hours, while phenobarbital lingers much longer, around 100 hours. This layered effect helps maintain steadier seizure protection throughout the day.
Essential Tremor
Outside of epilepsy, primidone is one of the two most widely used treatments for essential tremor, a condition that causes involuntary shaking, most often in the hands. If you’ve been told you have essential tremor and a beta-blocker like propranolol isn’t enough or isn’t an option, primidone is typically the next choice.
The evidence for this use is solid. In clinical studies, a single 250 mg dose reduced tremor amplitude by about 60% within one to seven hours. That effect came from primidone itself, not from phenobarbital, since blood tests showed no detectable phenobarbital levels during that window. This matters because it means primidone has its own direct anti-tremor action, separate from its breakdown products. Doses for essential tremor generally range from 50 to 1,000 mg per day, and the medication works in people who have never tried treatment as well as those already taking propranolol.
How You Start Taking It
Primidone is almost never started at the full dose. Instead, it follows a gradual step-up schedule over about 10 days to help your body adjust and reduce early side effects. For adults and children 8 and older, the typical pattern looks like this:
- Days 1 to 3: 100 to 125 mg once at bedtime
- Days 4 to 6: 100 to 125 mg twice a day
- Days 7 to 9: 100 to 125 mg three times a day
- Day 10 onward: 250 mg three times a day (maintenance dose)
For essential tremor, the starting dose is often even lower, sometimes just 25 or 50 mg at bedtime, since effective tremor control may require much less medication than seizure control. If you’re switching from another seizure medication to primidone, the transition is done gradually as well, with the new drug increasing while the old one tapers down.
Common Side Effects
The most frequent complaints when starting primidone are drowsiness, dizziness, and unsteadiness (sometimes called ataxia). These tend to be worst in the first few days and often improve as your body adjusts. That’s exactly why the dose starts low and increases slowly. Nausea can also occur early on.
Because primidone converts to phenobarbital in your body, it shares some of phenobarbital’s sedating qualities. You may feel noticeably sleepy or mentally foggy, especially at higher doses. This effect usually becomes less pronounced over weeks of regular use as tolerance develops, but some people continue to find it sedating. Coordination problems and fatigue are also possible at maintenance doses.
Long-Term Effects on Bone Health
One risk that often goes unmentioned in short-term discussions is what happens with years of use. Primidone speeds up the way your liver processes vitamin D, which can leave you with lower vitamin D levels over time. Less vitamin D means your body absorbs less calcium, and that gradually weakens bones.
Long-term primidone use is associated with decreased bone mineral density, which can progress to osteopenia, osteoporosis, and a higher risk of fractures. It can also cause osteomalacia, a condition where bones become soft due to poor mineralization. A UK government safety review confirmed that primidone, along with several other seizure medications, carries this risk. Vitamin D supplementation is generally recommended for anyone taking primidone long-term, particularly those already at risk for bone loss (older adults, postmenopausal women, people with limited sun exposure).
Who Should Not Take Primidone
Primidone is not safe for people with porphyria, a group of rare conditions affecting how your body makes heme, a component of red blood cells. The medication can trigger severe, potentially dangerous porphyria attacks. It’s also contraindicated if you’ve had an allergic reaction to primidone or to phenobarbital, since your body will produce phenobarbital as it processes the drug.
Because of its sedating properties, primidone interacts significantly with alcohol and other central nervous system depressants. Combining them amplifies drowsiness and impairs coordination. Primidone also interacts with many other medications because of how it affects liver enzymes. It can speed up the breakdown of birth control pills, blood thinners, and other drugs, potentially making them less effective. If you take primidone, your prescriber will likely need to check for interactions with anything else in your medication list.
Blood Level Monitoring
Because primidone produces phenobarbital as a metabolite, blood tests for this medication typically measure levels of both substances. Monitoring helps ensure you’re in a range that controls seizures or tremor without excessive side effects. This is especially important because phenobarbital accumulates slowly due to its long half-life (around 100 hours), meaning it can take weeks for blood levels to fully stabilize after a dose change. Your doctor may order periodic blood draws, particularly when starting treatment, adjusting doses, or adding new medications.