Welcome @miep, I'm sorry to hear that you had an adverse effect from the Pfizer vaccine. Hopefully members who have tried Clonazepam for BFS can share their experience with you. I did a search of Connect and only found one mention in this discussion by @captainanxiety8 here - https://connect.mayoclinic.org/comment/616427/. He shared another post on things that helped his BFS here - https://connect.mayoclinic.org/comment/312187/.

Here is some information I found on BFS that you might find helpful.

"Treatment for Fasciculations
Although some control of severe fasciculations can be achieved with beta-blockers and anti-seizure medications, no drug has ever been shown to alleviate the symptom consistently.

One of the most effective means of controlling BFS is relaxation and anxiety management. Anxiety has a cause-and-effect relationship with fasciculation; it can trigger an episode and aggravate its severity once it starts.

If anxiety is severe, it is best to seek help from a psychologist or psychiatrist who can provide stress-reduction training or prescribe anti-anxiety drugs if needed. Stimulants like caffeine should also be avoided."
-- Benign Fasciculation Syndrome: Causes and Treatment: https://www.verywellhealth.com/benign-fasciculation-syndromes-2488771

Did you report your adverse reaction to the COVID vaccine to VAERS ? - https://vaers.hhs.gov/reportevent.html