I wonder about the dosage you might’ve been taking. Many, many people take much more than needed, and higher doses are more likely to have adverse effects. 250-300 µg is an appropriate dosage for most adults. Note that common over-the-counter dosages are 10 to 25 mg. There is a Cochran collaboration meta-analysis supporting this.

I work in a pharmacy school in the US. One of my colleagues told me a funny anecdote about traveling in Great Britain. He had forgotten his melatonin, so went to a pharmacy to get some. The pharmacist told him that it was only available with a prescription, being a neurohormone. But here’s some promethazine OTC. That’s Rx only in the US.