Can foot drop and mononeuritis multiplex be solely caused by low vitamin b12?

Not typically: Vitamin B12 along with vitamin b6 and Folic Acid have a crucial role in nerve function. Low or high levels of vitamin B12 can cause symptoms affecting the nervous system; however, to have a one-sided foot drop and a mono neuritis (one nerve) multiplex is not as common - it's usually systemic (overall). Talk to your primary care physician for a complete work-up, a possible referral to neurology.