Those who have vitamin B12 deficiency are given this Methnex-Plus injection. This is a supplement drug that is also given to diabetic patients. Deficiency of the vitamin B12 is caused when people do not take products that are rich in this vitamin, like eggs, fish, meat and milk products. Not just this medication can treat the deficiency, but also prevent the deficiency and its causes, like anemia and nerves damage. Immediate action can be expected from the injectable drug. The common side effects of this medication are vomiting, skin rash, nausea, headache, to name a few.