From the Research
Antihistamines generally do not worsen myoclonus and may even help in some cases, but individual responses can vary. For most people with myoclonus, taking common over-the-counter antihistamines like diphenhydramine (Benadryl), cetirizine (Zyrtec), or loratadine (Claritin) at standard doses is unlikely to exacerbate myoclonic jerks, as supported by the lack of direct evidence linking antihistamines to myoclonus exacerbation in studies such as 1 and 2. In fact, some antihistamines with sedating properties, like hydroxyzine, are occasionally prescribed to help reduce myoclonus in certain patients.
That said, a small subset of individuals may experience increased muscle twitching or jerking as a side effect of antihistamines, particularly first-generation antihistamines that cross the blood-brain barrier more readily, as noted in 3. If you notice your myoclonus worsening after taking an antihistamine, discontinue use and consult your doctor. The reason antihistamines don't typically worsen myoclonus is that they primarily affect histamine receptors, which are not directly involved in the neural pathways that cause myoclonic jerks, as discussed in 4 and 5. Myoclonus is more closely related to abnormalities in GABA, glycine, and serotonin neurotransmitter systems.
If you need to take an antihistamine while managing myoclonus, start with a non-sedating second-generation option like cetirizine or loratadine at the lowest effective dose, as recommended in 3. Monitor your symptoms closely and report any changes to your healthcare provider. It's also important to note that some studies, such as 2, discuss drug-induced myoclonus, but antihistamines are not commonly implicated. Therefore, the use of antihistamines in patients with myoclonus should be approached with caution, but they are not generally considered a risk factor for exacerbating myoclonus, as supported by the highest quality and most recent study available, 1.