Terramycin can linger in the system for a long time
this is from The BarnWorld and Farm and Ranch Superstore page on Terramycin powder
http://www.barnworld.com/sa/p/TERRAMYCIN_SOLUBLE.htm
QUESTION:  I see that you are not supposed to eat eggs from chickens that have been given terramycin. Can the chicken be butchered and eaten after being given terramycin. If so, what is the waiting period. Also, if I have started giving the chicken terramycin (I just started today.), but want to switch to vet rx so that I can eat the eggs, would that be possible? If so, can I stop giving terramycin and then start with the Vet Rx or would there be some type of drug interaction? Thank you! 
ANSWER:  There could be a trace amount left in the yolk of the egg. It can be especially hard on someone prone to allergies.  
QUESTION:  Does this mean that the eggs can never be eaten form this chicken although has completed the regimen of the teramycin and may even be a few months passed the initial treatment. 
ANSWER:  It is recommended that you do not consume the eggs at any time. There can be a trace amount of Terramycin left in the yolk of the egg and if someone is prone to allergies they might have a reaction. 
QUESTION:  can you eat chickens that were given terramycin hci in the water. what effect if any does it have on humans 
ANSWER:  There is a possible trace amount of the drug that can be in the yolk of the egg, so it is recommended not to eat eggs from chickens that have been given Terramycin. This is especially important if someone is prone to allergies. 
QUESTION:  If you give this to chickens laying eggs how long do you wait to eat the eggs? 
ANSWER:  It is recommended not eating the eggs at any time of a bird that has been given Terramycin. There is a trace amount left in the yolk. 
I think "never" is a little extreme, but a safe withdrawal period would be 30 days.