There's no risk in non-smokers if you stick to a sane dose. Megadosing it can harm sleep and severely increase blood pressure and heart rate, although these gradually reverse upon cessation. This can take time to manifest. It is why I limit the dose to 500 mcg per day. Older people, such as those over 70, can need more and tolerate more, even 5 mg per day, due to deteriorated absorption.

Also, don't forget the other B vitamins. In truth I take them all.