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If you are sterile then there could be associated hormonal disorder where there is less testosterone
which can affect libido. Since you are hypertensive and diabetic even these can cause erectile dysfunction
. Also the drugs taken for controlling hypertension
can cause erectile dysfunction. If Viagra isn't working probably you need alternative treatment like pumps or surgical intervention to help achieve erection. Consult your Urologist
in this regard for the various options you have.