Lacosamide Side Effects: Vision, Heart, and Dose Risks

Lacosamide, an antiseizure medication sold under the brand name Vimpat, causes side effects that are overwhelmingly neurological in nature and strongly tied to dose. Dizziness is the most consistently reported problem, showing up across every dose level studied, while effects like nausea, double vision, and coordination trouble tend to emerge as the dose climbs. The drug also has a distinct cardiac signature worth knowing about, and a handful of rare but serious reactions that deserve attention even though most people never encounter them.

The Most Common Side Effects and How Dose Shapes Them

A systematic review and meta-analysis pooling data from randomized controlled trials found that of 21 identified adverse events, just over half were significantly linked to lacosamide, and the number of problems grew in a strikingly stepwise pattern with dose. At 200 mg per day, only dizziness stood out as significantly more common than placebo. At 400 mg per day, the list expanded to six: dizziness, vertigo, abnormal coordination, abnormal vision, nausea, and vomiting. At 600 mg per day, it widened further to nine, adding ataxia (unsteady gait), balance problems, double vision, fatigue, and tremor.¹ That dose-dependent pattern is one of the more useful things to understand about lacosamide: many of the side effects people experience at higher doses simply do not occur, or occur far less often, at the lower end of the dosing range.

Among the common side effects reported at rates of roughly 10% or more across clinical trials, dizziness, ataxia, double vision, and drowsiness tend to dominate.² These are all central nervous system effects, which makes sense given how lacosamide works. The drug targets sodium channels in nerve cells, but in a way that’s distinct from older seizure medications. Rather than interacting with the fast-acting phase of sodium channel activity, lacosamide selectively enhances what’s called “slow inactivation,” essentially making overexcited neurons harder to fire repeatedly.³ That mechanism calms seizure activity, but it also explains why the most frequent side effects involve the brain: dizziness, sleepiness, and problems with balance and vision are all signs that neural signaling is being dampened a bit too aggressively.

Heart Rhythm Changes

One of the more distinctive side effects of lacosamide compared to other seizure drugs is its effect on the heart’s electrical conduction. Specifically, the drug can lengthen the PR interval on an electrocardiogram, a measure of the time it takes for electrical signals to travel from the upper to the lower chambers of the heart. In pooled clinical trial data, this effect was small and dose-related: average PR interval increases were about 1.4 milliseconds at 200 mg/day, 4.4 milliseconds at 400 mg/day, and 6.6 milliseconds at 600 mg/day, compared to essentially no change with placebo.⁴ For the vast majority of people, these shifts are clinically meaningless. First-degree atrioventricular (AV) block, a mild conduction delay, was reported as a non-serious event in under 1% of patients in those same trials, and no second-degree or higher AV block was observed in the controlled trial population.

The picture looks a little different in real-world settings involving rapid intravenous loading, which is sometimes done in hospital for patients having frequent seizures. In one study of patients who received lacosamide by IV loading, the average PR interval increased from about 169 milliseconds to about 185 milliseconds, and new first-degree AV block appeared in 19 patients. A few patients developed atrial fibrillation or bradycardia. Older age was a significant risk factor for a bigger PR interval jump.⁵ There are also scattered case reports of second-degree AV block occurring in individual patients on lacosamide, including at least one case in a young, otherwise healthy athlete.⁶ These cases are rare, but they are why prescribing information recommends an ECG before starting treatment in anyone with known cardiac conduction problems and why clinicians keep an eye on heart rhythm when doses change.

Rare but Serious Reactions

Beyond the common neurological complaints and the heart rhythm question, lacosamide has been associated with a few uncommon but potentially serious adverse effects that are important to recognize early.

DRESS syndrome: Drug Reaction with Eosinophilia and Systemic Symptoms is a severe immune-mediated reaction that can cause widespread rash, fever, liver inflammation, and abnormal blood counts. It has been reported with lacosamide, and there’s particular concern about cross-reactivity in people who have already developed DRESS from other antiseizure drugs like phenytoin or phenobarbital. One published case described a patient who developed DRESS rapidly after starting lacosamide, possibly because the drug shares an aromatic ring structure with the medications that triggered the original reaction.⁷ Expert reviews have flagged that lacosamide, along with eslicarbazepine and cenobamate, should be used cautiously in people with a history of drug allergies.⁸
Psychiatric effects: Depression and suicidal ideation have been reported in association with lacosamide use. In one published case, a patient developed sudden depression and suicidal thoughts shortly after starting lacosamide, and the symptoms reversed completely once the drug was stopped.⁹ This is consistent with a class-wide concern for antiseizure drugs: the FDA has long required a warning about increased suicidality risk for the entire category. A review of lacosamide’s safety profile also noted psychological symptoms and suicide risk among the adverse effects that have been emphasized in the literature.¹⁰
Blood and skin reactions: Skin rashes and hematotoxicity (abnormalities in blood cell production) have also been reported, though they remain uncommon.¹¹

None of these serious events are common enough to show up in large randomized trials as statistically significant signals. They emerge in case reports, post-marketing surveillance, and safety reviews. But their severity means that any new rash, unexplained fever, mood change, or unusual fatigue shortly after starting or increasing lacosamide warrants prompt medical evaluation.

Interactions With Other Sodium Channel-Blocking Drugs

If you take lacosamide alongside another antiseizure medication that also works on sodium channels, such as carbamazepine, oxcarbazepine, or phenytoin, you may be more likely to experience neurological side effects like double vision, dizziness, and drowsiness. This is not a traditional pharmacokinetic interaction where one drug raises the blood level of the other. In a study of 39 patients who developed these neurotoxic symptoms on lacosamide combination therapy, researchers identified a subset in whom blood levels of the other drugs had not changed at all. The symptoms resolved when the dose of the other sodium channel blocker was reduced, suggesting a purely pharmacodynamic interaction: both drugs suppress sodium channel activity, and the combined effect is more than either one alone.¹²

This interaction has practical consequences for how well people tolerate lacosamide long term. A large study of children with epilepsy found that those taking a sodium channel blocker alongside lacosamide reported side effects more often (65% versus 39%) and were roughly twice as likely to stop lacosamide entirely, even after accounting for the side effects themselves.¹³ The takeaway is not that the combination cannot work, but that dosing both drugs on the lower side and paying attention to early warning signs of neurotoxicity matters more in this scenario.

How Lacosamide Compares on Tolerability

When researchers have tried to rank the newer antiseizure drugs by tolerability, lacosamide generally falls in the middle tier. A network meta-analysis comparing withdrawal rates across randomized trials found that drugs like brivaracetam, gabapentin, and levetiracetam had significantly lower rates of dropout due to adverse events, while lacosamide, eslicarbazepine, oxcarbazepine, and topiramate had higher withdrawal rates.¹⁴ That does not make lacosamide poorly tolerated in absolute terms; it means some alternatives are easier to stay on for some people. Because tolerability is so individual, knowing lacosamide’s specific side-effect profile helps you have a more informed conversation with your doctor about what to expect and when a switch might make sense.

Vision-Related Effects

Double vision (diplopia) is one of the classic side effects of lacosamide, particularly at higher doses, and it was flagged in the meta-analysis as significant at the 600 mg/day level. Beyond diplopia, a large-scale analysis of the FDA’s adverse event reporting system found that lacosamide rarely causes severe eye-related problems overall. The only eye disorders reaching statistical significance in that analysis were diplopia and metamorphopsia, a distortion of visual shapes.¹⁵ Compared to some other antiseizure drugs, lacosamide’s eye safety profile is relatively clean.

One unusual visual phenomenon worth mentioning is downbeat nystagmus, an involuntary downward beating of the eyes. Case reports have linked lacosamide to this condition, likely because the drug’s effect on sodium channels in cerebellar Purkinje neurons can disrupt the part of the brain responsible for gaze stability.¹⁶ It is rare, but if you notice that objects seem to be drifting or bouncing while you look at them, it is worth reporting to your neurologist.

Side Effects in Older Adults

Older adults tend to be more sensitive to the neurological side effects of antiseizure drugs in general, and lacosamide is no exception. In a study of patients averaging 71 years old, about 60% stayed on lacosamide over a follow-up period averaging nearly two years. Among those who stopped because of side effects, dizziness and gait instability accounted for more than a third of the discontinuations.¹⁷ The cardiac conduction effect is also more relevant in older patients. As noted earlier, older age was a significant predictor of larger PR interval changes after IV loading.¹⁸ Because older adults are more likely to have pre-existing cardiac conduction disease, ECG monitoring around the time of dose changes is especially prudent in this group.

Side Effects in Children

The side-effect landscape in children overlaps with adults but has some differences in emphasis. A systematic review and meta-analysis of pediatric studies found that the most common adverse events were drowsiness (about 15%), dizziness (about 10%), and somnolence (about 8%).¹⁹ A separate observational study of children with drug-resistant epilepsy reported that side effects occurred in about half the patients, with somnolence as the leader at 18%, followed by behavioral changes (about 16%), headache, and dizziness.²⁰ The behavioral changes are worth noting because they are less prominently discussed in the adult literature; they include irritability and agitation, which can be easy to mistake for something unrelated to the medication in a young child.

In very young children under age four, a prospective study from China found adverse events in about 14% of patients, including somnolence, irritability, vomiting, difficulty walking, fatigue, and one skin rash. Reassuringly, no cardiac arrhythmias were detected among the 60 children who had ECG monitoring during follow-up.²¹

Kidney and Liver Impairment

Lacosamide is cleared from the body through both the kidneys and the liver, so impairment of either organ changes how the drug behaves. In healthy volunteers, the kidneys account for about 30% of total clearance. With severe kidney impairment, that renal contribution drops to roughly 11%, and the drug’s half-life and overall exposure in the bloodstream rise.²² An animal study confirmed that both experimentally induced liver and kidney impairment increased blood levels of lacosamide and altered its effects, suggesting that these conditions could affect both the drug’s safety and effectiveness.²³

Pharmacokinetic modeling has attempted to quantify how much dose adjustment is needed. One model recommended reducing the dose to roughly 75% of normal in moderate kidney impairment and to about 63% in severe impairment. For liver disease, the reductions are steeper: down to about 72% in moderate hepatic impairment and about 36% in severe hepatic impairment.²⁴ The practical point is that if you have significant kidney or liver disease, the same dose of lacosamide results in higher drug levels, which means a higher chance of hitting the side-effect threshold. Your doctor will likely start lower and titrate more slowly.

Pregnancy

Data on lacosamide during pregnancy is still relatively limited compared to older seizure medications, but what exists is cautiously reassuring. An observational study analyzing 55 prospectively tracked pregnancies exposed to lacosamide did not find an increased risk of major birth defects or miscarriage.²⁵ A separate analysis of spontaneous and solicited reports found congenital malformations in about 2.3% of pregnancies exposed to lacosamide alone, compared to about 6.9% in pregnancies where lacosamide was taken alongside other antiseizure drugs.²⁶ The monotherapy rate is within the range typically seen in the general population. The polytherapy number is harder to interpret because those pregnancies involved multiple medications, making it difficult to attribute risk to lacosamide specifically. No one would call these datasets large enough to be definitive, but they do not raise the kind of red flag seen with drugs like valproate. If you are of childbearing potential and taking lacosamide, the conversation with your neurologist should weigh seizure control against this evolving but so far reassuring safety picture.

Abuse Potential and Stopping the Drug

Lacosamide is a Schedule V controlled substance in the United States, the lowest classification for controlled drugs. That scheduling was based in part on a study that gave lacosamide to recreational users of central nervous system depressants and compared their subjective experience to alprazolam (a benzodiazepine) and placebo. Lacosamide did produce detectable abuse-related subjective effects, but they were relatively weak compared to alprazolam.²⁷

When it comes to discontinuation, studies have not found that stopping lacosamide suddenly produces the kind of unpleasant physical withdrawal symptoms that can occur with benzodiazepines or opioids.²⁸ That said, stopping any antiseizure medication abruptly can trigger breakthrough seizures, so tapering is still the standard approach. The low abuse potential and absence of a recognized withdrawal syndrome distinguish lacosamide from many other drugs that act on the central nervous system.

Intravenous Administration

Lacosamide is available in both oral and intravenous formulations, and the IV version is sometimes given as a rapid push rather than the standard slower drip infusion. For patients and families who encounter lacosamide in a hospital setting, it is worth knowing that studies comparing fast IV push to the conventional slower piggyback infusion have found similar rates of adverse events between the two methods. One study reported infusion-site reaction rates of about 1.8% with push dosing versus 0.8% with the slower infusion, a difference that was not statistically significant, and nearly every adverse event was clinically insignificant.²⁹ Another study found similar rates of low blood pressure and slow heart rate between the push and drip groups.³⁰ In urgent situations, clinicians can generally give lacosamide IV quickly without introducing a meaningfully different set of risks.