Lacosamide, sold under the brand name Vimpat, is a seizure medication whose most common side effects are dizziness, headache, nausea, and double vision. These tend to be dose-related, meaning they’re more likely at higher doses and often improve as your body adjusts. But lacosamide also carries some less obvious risks, including effects on the heart’s electrical system and a small increase in suicidal thoughts, that are worth understanding before you start or while you’re taking it.
The Most Common Side Effects
The side effects people experience most frequently with lacosamide overlap with many other seizure medications. Dizziness and drowsiness top the list, and they’re the reasons most people discontinue the drug in clinical trials. Headache, nausea, and double vision (where you see two overlapping images) are also reported frequently. These effects are tied to how the drug works: lacosamide slows down overactive electrical signals in the brain by targeting sodium channels on nerve cells, but that same dampening effect can make you feel unsteady, foggy, or visually off-balance.
Other commonly reported effects include fatigue, blurred vision, tremor, difficulty with coordination, and vomiting. In children, the profile looks similar: dizziness, sleepiness, headache, blurred vision, upset stomach, diarrhea, dry mouth, and excessive sweating have all been noted. Most of these side effects are milder at lower doses and tend to ease within the first few weeks of treatment, especially when the dose is increased gradually.
Effects on Heart Rhythm
One of lacosamide’s more distinctive risks is its effect on the heart’s electrical conduction system. Specifically, it can lengthen something called the PR interval, which is the time it takes for an electrical signal to travel from the upper chambers of the heart to the lower chambers. A longer PR interval doesn’t always cause symptoms, but in some people it can lead to abnormal heart rhythms or a feeling that the heart is beating too slowly.
In one study of critically ill patients receiving intravenous lacosamide, about 13% had a clinically meaningful increase in their PR interval after just the first dose, and 14% shifted from a normal reading to an abnormally high one. This is particularly relevant if you already have a heart condition, take other medications that slow heart conduction, or have a history of fainting spells. Your doctor may order an ECG (a quick, painless heart tracing) before starting lacosamide and periodically afterward to monitor for changes.
Mood Changes and Suicidal Thoughts
All seizure medications, lacosamide included, carry an FDA-boxed warning about a small but real increase in suicidal thoughts or behavior. Pooled data from 199 placebo-controlled trials of various seizure drugs found that treated patients had roughly twice the risk of suicidal thinking compared to those taking a placebo. In practical terms, that works out to about 1 additional case for every 530 people treated, or approximately 4.3 per 1,000 patients versus 2.4 per 1,000 on placebo.
Among people taking these medications specifically for epilepsy, the relative risk was higher (3.5 times that of placebo), though the absolute numbers remained small: about 3.4 per 1,000 epilepsy patients on the drug versus 1 per 1,000 on placebo. These thoughts can emerge within the first few weeks of treatment. If you notice new or worsening depression, anxiety, agitation, irritability, or any thoughts of self-harm, that’s something to bring up with your prescriber promptly rather than waiting for a scheduled visit.
Allergic and Hypersensitivity Reactions
Rarely, lacosamide can trigger a serious allergic reaction involving multiple organ systems. The most concerning form is sometimes called DRESS (Drug Reaction with Eosinophilia and Systemic Symptoms), which typically starts with a widespread skin rash and fever, then progresses to involve the liver, kidneys, or lymph nodes. It usually appears two to eight weeks after starting a new medication, not immediately.
Warning signs include a spreading rash (especially one with blisters or peeling), facial swelling, swollen lymph nodes, persistent fever, or yellowing of the skin or eyes. This reaction is rare but can be life-threatening, and it requires stopping the medication immediately.
Why You Shouldn’t Stop Abruptly
If you need to stop taking lacosamide for any reason, including side effects, it should be tapered over at least one week rather than stopped suddenly. Abruptly discontinuing any seizure medication can trigger a rebound increase in seizure frequency, including potentially dangerous prolonged seizures. This applies even if you feel the medication isn’t working well or if side effects are bothersome. Your prescriber can set up a tapering schedule that minimizes this risk.
Who May Be More Sensitive to Side Effects
Certain groups tend to experience lacosamide’s side effects more intensely. People with kidney or liver problems may process the drug more slowly, leading to higher levels in the bloodstream and a greater chance of dizziness, nausea, or cardiac effects. Older adults are generally more susceptible to the balance and coordination problems that lacosamide can cause, which raises fall risk. And anyone already taking medications that affect heart conduction (certain blood pressure drugs, some antidepressants) faces a compounded risk of PR interval prolongation when lacosamide is added.
If you’re experiencing side effects that feel more than mild or that aren’t improving after a few weeks, it’s worth discussing whether a dose adjustment or alternative medication might work better. Many of lacosamide’s side effects are dose-dependent, so even a modest reduction can make a noticeable difference in how you feel day to day.