found this in an old post:
"Maximal plasma HCG - human chorionic gonadotropin - levels will be reached in males approximately 6 and 16 hours after a single intramuscular or subcutaneous injection of HCG - human chorionic gonadotropin - , respectively and in females after approximately 20 hours. HCG - human chorionic gonadotropin - is approximately 80 per cent metabolized, predominantly in the kidneys. Intramuscular and subcutaneous administration of HCG - human chorionic gonadotropin - were found to be bioequivalent regarding the extent of absorption and the apparent elimination half-lives of approximately 33 hours. On basis of the recommended dose regimens and elimination half-life, cumulation is not expected to occur."
Looks like the main difference between subq and IM, is the amount of time for blood levels to peak, 6hrs for IM, 16hrs for subq.