can you overdose on vitamin b12 injection Take Too Much B12? and Side Effects The Dangers of Vitamin B12
Description
The conclusion is that human sinus and AV nodes are sensitive to physiologic doses of adenosine and this injection can be safely and effectively used to terminate acute episodes of supraventricular tachycardia.

Intake of caffeine products can limit the Adenosine function and might lead to no successful effect
Dosage and Schedule: A severe deficiency may require a "loading dose" of frequent injections to rebuild your body's stores

Yes, when administered by a trained professional, they are safe

Research institutions implementing this 5-amino-1mq dosage protocol measure potency via HPLC (high-performance liquid chromatography) at batch receipt and again at 14-day intervals
Ziegler,L.J