Hi, i was wondering the following;”:
1. What is the half-life of a b12 (hydro) in plasma?
2. How long can the body use the remaining b12 (of the injection) for repair?
3. Suppose the following: the half-life is 1 week. Are the following (plasma) levels right?
Week 1: 1ste injection, total level is 1
Week 2: 2nd injection, total level is 1.5
Week 3: 3rd injection, total level is 1.75
Week 4: 4th injection, total level is 1.875
I think I recall that fbirder answered a question on the subject of half life in the last month or two so it might be worth having a search.
With no scientific or medical background I would suggest that in answer to your point 2 that it depends on the individual body. For example, how slowly or quickly an individuals kidneys clear out excess B12.
B12 is not radioactive and so it does not have a half-life. Also it is water soluble and cannot be stored.
It gets converted into various forms and gets used up by cells. The larger the concentration in the blood, they more that is available for the cells.
The liver is supposed to provide a constant trickle of B12 in the blood and the rest it dumps into the bile. The kidneys also clean it out of the blood.
If the recycling mechanism is working in the gut, it gets reabsorbed by the ilium and is sent back to the liver in the hepatic loop.
The concentration in the blood with a flawed recycling mechanism is an exponential decay curve where the rate of change is proportional to the concentration in the blood. This is probably what you mean by a half life.
The rate of exponential decay is a function of activity and stress as the cells use it up.
IM or subcutaneous injections attempt to store an analogue form of B12 in muscle or fat and again the amount that filters out to the blood stream is an exponential decay curve. Exercises can push a bit more out of the muscle of fat (as rolling a tube of toothpaste does for the last remnants.)
For me, the B12 IM injection lasts about three days.
Thank you for your reply! While half-life is a technical term used in radioactive materials is also used in medicines to mean how long does it take for the body to break down the active ingredient of the product to half its rate of total potency. It is used to determine the frequency of retreatment of a product and, in farmed animals, the calculation of theoretical "withdrawal" times (multiple times the half-life) of meat and milk between administration and safety for consumption.
just to echo what deniseinmilden has said - 'half-life' is used in medicine - and doesn't just relate to radio-activity. Some drugs have very long half-lives (stay in the body/blood for a long time) others have a much shorter half-live. This affects the dosage and frequency of dosage and also how easy it is to manage a condition using particular medications. Using drugs with a long half-life can mean long lead times to getting doses correct ... and they are unlikely to be suitable in patients where the condition isn't stable.
To be honest its not a particular useful concept in relation to treatment for B12 absorption problems - because the retention mechanisms for B12 are complex and vary so much from individual to individual ... and different mechanisms seem to come into play in some individuals as serum B12 levels vary.
I have seen it quoted that it is 6 days in the blood stream and 400 days in the liver so in theory your assumption is correct but in practice it doesn't work like that - everything is way more complicated and varies from person to person! 😁
My test results help please
B12 frequency
B12 high 
