There are five causes of a man's penis shrinking.
The average measurement of the organ is around 13.12cm long with a girth of 11.66cm when erect.
Smoking is one of the main reasons as it can damage the blood vessels and can even cause problems with getting an erection.
Other things on the list, compiled by The Sun Online, include medicines - particularly those to treat prostate problems - as well as getting older.
As men age, their penis and testicles can shrink due to the build-up of fatty deposits in the arteries, which can reduce blood flow.
The last two include prostate surgery - with up to 70 per cent of men who have had the surgery to remove the cancer gland experience a reduction.
The last one is Peyronie's disease, which is a condition that causes the penis to curve when it's erect.
However, it can also lead to scar tissue which can block blood flow and make it smaller when hard.