Vitamin B12 plays essential roles in red blood cell formation, cell metabolism, nerve function and the production of DNA. Vitamin B12 impact on the body’s vital functions means that lacking it can cause a wide range of symptoms. These usually develop gradually, but can worsen if the condition goes untreated.While there are common symptoms associated with a vitamin B12 deficiency, there are a number of symptoms that may signal a borderline vitamin B12 deficiency.A borderline B12 deficiency is to be taken just as seriously as a full-blown deficiency, as the Thyroid Patient Advocacy (TPA) website explained: “Borderline B12 deficiency can sometimes cause symptoms so dramatic that B12 deficiency may be rejected as a possible cause because it may be thought that such symptoms wouldn’t be caused by a mere borderline deficiency.B12 deficiency sometimes goes undiagnosed until the symptoms become moderate to severe, although this is not necessarily the fault of the medical profession, notes the health body.It is therefore imperative to spot the warning signs associated with a borderline B12 deficiency because the longer you leave it, the greater the risk of developing permanent complications.READ MORE: Vitamin D deficiency symptoms: Experiencing this feeling is a warning signBorderline B12 deficiency: A sharp stabbing or tingling pain in the hands is a warning sign (Image: Getty Images )One warning of a B12 deficiency is a sharp stabbing, tingling pain in the palm of one or both hands.According to the TPA, this occurs suddenly and for no apparent reason in a spot directly below the ring finger, approximately where the first palm crease is.“If B12 deficiency is not treated, a tingling pain may begin to occur along the outside edge of the hand, starting from the wrist. This pain occurs when the wrist is flexed backward,” explained the health site.Other borderline symptoms include:Shortness of breath, but without chest painEye twitchFacial pain, usually on only one side of the face at a timeTingling along the back of one or both thighsMigraine headachesDON’T MISS How to live longer: Best diet to improve your life expectancy – what foods to eat [TIPS]Lung cancer symptoms: Does your face look like this? Warning sign of the deadly disease [INSIGHT]How to lose visceral fat: The health drink proven to reduce the harmful belly fat [TIPS]More commonly associated symptoms include:Extreme tirednessA lack of energyPins and needles (paraesthesia)A sore and red tongueMouth ulcersMuscle weaknessDisturbed visionPsychological problems, which may include depression and confusionProblems with memory, understanding and judgementWhen to see a GPYou should see a GP if you think you may have a vitamin B12 or folate deficiency because these conditions can often be diagnosed based on your symptoms and the results of a blood test, explains the NHS.It is also important to get any symptoms checked out sooner rather than later because although many of the symptoms improve with treatment, some problems caused by the condition can be irreversible, warns the NHS.Neurological conditions such as vision problems or memory loss may be permanent in some cases.Vitamin B12 treatment: Injections are usually the first course of treatment (Image: Getty Images )How to treat a vitamin B12 deficiencyVitamin B12 deficiency anaemia is usually treated with injections of vitamin B12.There are two types of vitamin B12 injections:HydroxocobalaminCyanocobalaminIf your vitamin B12 deficiency is caused by a lack of the vitamin in your diet, you may be prescribed vitamin B12 tablets to take every day between meals.People who find it difficult to get enough vitamin B12 in their diets, such as those following a vegan diet, may need vitamin B12 tablets for life, explains the NHS. Vitamin B12: The vitamin can be found in foods such as eggs, meat and fish (Image: Getty Images )Although it’s less common, people with vitamin B12 deficiency caused by a prolonged poor diet may be advised to stop taking the tablets once their vitamin B12 levels have returned to normal and their diet has improved, says the health body.Good dietary sources of vitamin B12 include:MeatSalmon and codMilk and other dairy productsEggsFor vegetarian or vegans looking to top up the vitamin, there are alternatives to meat and dairy products, such as yeast extract (including Marmite), as well as some fortified breakfast cereals and soy products.“Check the nutrition labels while food shopping to see how much vitamin B12 different foods contain,” advises the NHS.