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The average penis measures somewhere between 8 to 12 centimeters when not erect, and 12 to 18 centimeters when erect.
A penis is considered abnormally small only if it measures less than 3 inches (or about 7 centimeters) when erect, a condition called micropenis.
So your size of penis of 12 cm when erect is normal.There is nothing of concern regarding the size.
Your main issue is problem in erection.This is medically called as ED(erectile dysfunction).Its most common causes are: ageing,cardiovascular diseases,cigarette smoking,,low testosterone levels,certain medications,depression and anxiety.
Stress,anxiety & depression are one of the most common cause of erectile dysfunction (ED).
You may need following tests:
-Dynamic infusion cavernosometry(DIC) and cavernosography (DICC),
-Nocturnal tumescence test.
The treatment depends on the cause.It includes Oral medications like Sildenafil (Viagra),Tadalafil (Cialis),life style improvements (for example; quitting smoking, and exercising more),testosterone therapy.
So my best advise for you is to make an appointment with your doctor to have these tests done so as to know the exact underlying cause & then treatment will be prescribed accordingly.
There is no need to worry.This condition is treatable very succesfully with above described treatments.
Hope this helped
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