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HCG injections are used in female infertility;delayed puberty;in males to increase sperm count
also.The common adverse effect seen in few females are:
-headache;tiredness;mood changes;pain at injection site;paripheral oedema;depression
but no low back pain
or pain in kidney
So the pain in low back may be due to some other reasons for which please consult your doctor and get treated and you should be fine.