Yes, there can be symptoms of high B12 levels in some people. If my level rises above 700 pg/mL I experience insomnia. It took me years to figure out what the culprit was. Fortunately, my multivitamin has just 100 mcg of B12 that keeps my level between 650-700 and lets me sleep soundly.

Just because there is no established upper limit for B12 it doesn’t mean there are no adverse effects. Repeated dogma dictates we will pass the excess out in urine. That’s not entirely true.

If you subscribe to ConsumerLab.com you will see there are many well researched and published side-effects of high B12 levels including:

1. Higher incidence of heart attack, stroke, death in diabetics.
2. Poorer disease-free and survival rates of those undergoing chemotherapy.
3. 77% increased relative risk of having colorectal cancer for those taking 500 mcg of B12.
4. 85% increased chance of early death above a level of 455 pg/mL
5. There are more adverse effects cited as well.
https://www.consumerlab.com/reviews/review-best-b-vitamins-and-complexes-energy-b6-b12-biotin-niacin-folic-acid/bvitamins/
Unless people have a B12 deficiency to correct it’s probably best to get our levels measured, stay with a low-dose B12 supplement if needed, and not let our levels get too high.