B12 has many functions in our body, so unfortunately a deficiency will give rise to a wide variety of symptoms, like brain fog, memory problems, cognitive impairment, insomnia, headaches, especially migraine, behavioural changes, learning problems, nominal aphasia, mood swings, irritability, depression, anxiety, hallucinations, delusions, psychosis, peripheral paraesthesia (pins and needles), numbness, neuropathic pains, poor balance, reduced vibration sense or proprioception (joint position sense), tinnitus, ataxia, taste impairment, sometimes myelopathy, fatigue, anaemia (either with larger red blood cells, or with normal size of the red blood cells when there is also iron deficiency), other reduction in blood cells, abdominal complaints, malabsorption, failure to thrive, weight loss, diarrhoea, hyperpigmentation, glossitis, (aphthous) stomatitis, infertility, urinary tract infections, joint and muscle pain, muscle weakness, spasticity, seizures, cardiomyopathy, urinary and/or faecal incontinence, postural hypotension/dizziness, erectile dysfunction, to name a few

Tsai YW, Kann NH, Tung TH, Chao YJ, Lin CJ, Chang KC, et al
Symptoms of B12 deficiency include: Low energy and fatigue Digestive problems Depression and moodiness Insomnia Brain fog and memory loss Anemia Reduced immune system function Tingling or numbness in the extremities Swollen tongue Loss of vision or hearing Jaundice Kidney disease Pale skin Poor circulation in the extremities If you are over age 50 and are experiencing one or more of the above symptoms, boosting your intake of B12 cannot hurt, and it may provide a safe and inexpensive solution while preventing a wrong diagnosis
Vitamin B12 is more effective as an injected than in pill form, so you get the maximum benefit of this nutrient
The sequencing of multiple peptide therapies affects overall outcomes