You mentioned that one of your cats has sarcoma -- so I am copying and pasting a section on B-12 and cancer in cats. You may not want to supplement that particular kitty until you read this and decide first (or check blood first to see if he/she is deficient in B-12.) There's information on both sides of the issue. I will also link the entire B Vitamin pages from the feline crf.org site here. It goes over every type of B vitamin and it's use in the body, forms of B vitamins (liquid, injections, etc.) and more!
http://felinecrf.org/vitamin_b.htm#methylcobalamin
Vitamin B12 and Cancer
Hypercobalaminaemia is associated with hepatic and neoplastic disease in cats: a cross sectional study(2014) Trehy MR, German AJ, Silvestrini P, Serrano G & Batchelor DJ
BMC Veterinary Research 10 pp175 looked at elevated cobalamin levels in cats who had not received vitamin B supplementation and states "The current study has suggested possible associations between hypercobalaminaemia and the presence of either solid neoplasia or liver disease in cats."
It may be wiser not to give vitamin B12 to a cat with cancer because cancer cells rely on vitamin B12 for growth, according to
Immunohistochemical quantification of the cobalamin transport protein, cell surface receptor and Ki-67 in naturally occurring canine and feline malignant tumors and in adjacent normal tissues(2015) Sysel AM, Valli VE & Bauer JA
Oncotarget 6 pp2331-2348, which states "Cancer cells have an obligate need for cobalamin (vitamin B12) to enable DNA synthesis necessary for cellular replication."
Purina Pro Club Update (2013)
12(1) Dr Sysel of the Bauer Research Foundation states "Vitamin B12 is important in DNA synthesis, as cells cannot divide without it. Rapidly growing tumors that are actively dividing have an especially high demand for vitamin B12. Tumor cells produce TCII [transcobalamin, a protein which moves vitamin B12 through the bloodstream] to obtain all the vitamin B12 they can."
However, researchers are also looking into whether they can use cancer cells' need for vitamin B12 to target cancer.
Immunohistochemical quantification of the cobalamin transport protein, cell surface receptor and Ki-67 in naturally occurring canine and feline malignant tumors and in adjacent normal tissues(2015) Sysel AM, Valli VE & Bauer JA
Oncotarget 6 pp2331-2348 adds "The potential to utilize these proteins as biomarkers to identify neoplastic tissues, streamline therapeutic options, evaluate response to anti-tumor therapy and monitor for recurrent disease has important implications in the advancement of cancer management for both human and companion animal patients."
A stability-indicating HPLC method for the determination of nitrosylcobalamin (NO-Cbl), a novel Vitamin B12 analog (2014) Dunphy MJ, Sysel AM, Lupica JA, Griffith K, Sherrod T & Bauer JA
Chromatographia 77(7-8) pp 581-589 reports further on the use of nitrosylcobalamin "as a biologic ‘Trojan horse’, utilizing the vitamin B12 transcobalamin II transport protein and cell surface receptor to specifically target cancer cells."
University of Pennsylvania School of Veterinary Medicine has been researching whether cats with
lymphoma might actually have a vitamin B12 deficiency (which is quite possible, because lymphoma may reduce vitamin B12 absorption in the intestines), and if they do, they plan to offer supplemental treatment to address the deficiency.