Is it intramuscular shots? Here's a link to some info:
dailymed.nlm.nih.gov/dailymed/archives/fdaDrugInfo.cfm?archiveid=5765"Blood concentrations of 0.003 units per mL may still be detectable 4 weeks following administration of 1,200,000 units."
Looks like it stays in the system for awhile - but in low concentration?