The answer starts with — “It depends on your DNA…” If you have night-sweats or unexplained fevers…. B12 may cause an improvement
- “Among individuals with B12 deficiency, the frequency of the TT MTHFR genotype was particularly high. The TT polymorphism was associated with endothelial dysfunction even after 6 weeks of treatment with B12 and folic acid.” 
- “treatment with 1 mg daily of sublingual vitamin B12 and 5 mg daily of folic acid for 6 weeks “
- Supplements are sold as mcg 1000 mcg = 1 mg
- ” received sublingual cobalamin preparation, 1000-mcg sublingual nuggets per day for 1 month. Normalization of serum cobalamin was obtained (790 pg/mL) after 1 month of treatment. ” 
- “Vitamin B12 treatment, which is simple, inexpensive and low-risk, seems to be effective for patients suffering from recurrent aphthous stomatitis, regardless of the serum vitamin B12 level.” 
- “girl with unusual appearance of periodic fever syndrome. … treatment with sublingual tablets of vitamin B12 (VIT B12). Beyond all expectation, we witnessed complete resolution of attacks during the first 2 months following treatment. After a long episode-free period, the parents withheld the treatment on their own accord, and in a short time the disease recurred. We advised to renew the treatment with VIT B12, and during approximately half year of follow-up there were no recurrences of periodic fever.” 
- “A dose of 500 micro g of cobalamin given either sublingually or orally is effective in correcting cobalamin deficiency.” 
- “Decreased serum vitamin B12 level in patients with type 2 diabetes who are treated with metformin can be corrected through treatment with either hydroxocobalamin injections or methylcobalamin sublingual supplements.” 
Response to vitamin B12 and folic acid in myalgic encephalomyelitis and fibromyalgia.
- “Frequent injections of high-concentrated vitamin B12, combined with an individual daily dose of oral folic acid, may provide blood saturations high enough to be a remedy for good and safe relief in a subgroup of patients with ME/FM. Moreover, we suspect a counteracting interference between B12/folic acid and certain opioid analgesics and other drugs which have to be demethylated as part of their metabolism. “
- ” Some patients can do well from oral treatment, but along the years we have recognized that patients with ME or FM respond best to the injective form of B12 therapy, and most of them require an injection at least once a week.”
- “methylcobalamin became available, in 2 mL ampoules with 5 mg/mL; ” i.e. 10mg per injection(once a week) –> 1.4 mg or 1400 mcg / day.
Do not use Laughing Gas(nitrous oxide) at the Dentist
- “the use of N2O has become increasingly popular in recent years. Unfortunately, the use of N2O leads to inactivation of vitamin B12. Vitamin B12 plays an essential role in the synthesis and maintenance of myelin, a fatty substance that surrounds nerve cells and is crucial for their functioning. Deficiency of vitamin B12 could typically result in degeneration of posterior and lateral columns of the spinal cord. Treatment with intramuscular vitamin B12 injections and abstinence of N2O generally leads to gradual improvement of symptoms” 
B12 / Folate Balance
The issue arises when B12 levels are low… from  review: “Simply put, if your vitamin B-12 status is good, folate supplementation is good for you!”
- “Those with a low vitamin B-12 status (serum cobalamin <148 pmol/L) and high serum folate (>59 nmol/L) had an odds ratio for cognitive impairment of 5 [times higher] compared with those whose vitamin B-12 status and folate status were both normal. “
- ” In agreement with current knowledge, they found that a low vitamin B-12 status is associated with macrocytosis, anemia, and cognitive impairment. “
- “The “good news” is that, in subjects with a normal vitamin B-12 status, high serum folate (>59 nmol/L) was associated with protection from cognitive impairment. This finding is remarkable in a population with a much higher mean folate concentration (39 nmol/L) than that seen in countries where there is no mandatory folate fortification. “
- “Is it possible that poor vitamin B-12 status influences the metabolism of folic acid or the function of NK cells after high intakes of folic acid?”
Oral, sublingual or injections are equally effective. If there is a cost for injections, then going to oral or sublingual will get you the same results without the expense.
1000 mcg/ 1 mg seems to be the effective ongoing dosage — especially if you have night sweats or unknown fevers.
Methylcobalamin or cobalamin? This depends on your MTHFR status. This blog post on 23 and me, is worth reading. There is no evidence that methylcobalamin have any adverse risk, then that would be preferred. On Amazon, you can get 1000 mcg or 5000 mcg for about $0.06 each (cost per day).