how do injnection work inside our bodies

routes of injection are : 

1- intra muscular ( i.m )

2- intravenous (i.v)

3- subcutaneous ( s.c) 

what happenes in every way by simple way 

first :

intra muscular : after injection the substances of the drug is absorbed by the blood vessels and circulate in the blood stream...... then making it is desirablew action 

as : most antibiotic and anti inflammatory 

second : intravenous :

 the injected substabnces is directly in the blood stream so it do its action by arapid way

third : the sub cutaneous 

the injected  drug also absorbed fast by blood vessels also it could be absorbed the epithelial cells and then go to blood stream and make its affect 

as : insulin injection 

there are several other ways of injections but not very commons as the above three method