Is this Flexeril by any chance? I was prescribed it three decades ago for a sore back, and it made me exceedingly sleepy for about an hour afterwards. Took maybe half an hour after ingestion, with supper, and then I was bonked for at least an hour. But, I don't recall it also making my heart race. I didn't notice it if it did.

Maybe try half a tablet if it is a tablet. Wait for a few hours, and if you get the desired effect, or halfway there, but no racing heart, you should be okay? I would never counsel you to double a dose of a drug, but halving it as a trial might help to alleviate any fears.