A penis pump is a cylinder that is fitted over the penis, with a manual or motorized pump to create suction. As the apparatus creates a partial vacuum around the penis, blood is drawn into the penis, helping it to become engorged.
As vacuum increases, the difference between the inner blood-pressure and the pump pressure increases as well; excessive pressure causes vascular damage rather than a harder penis.
Penis pumps, usually described in this context as vacuum pumps, have use in conventional alleviation of impotence. The pump itself is essentially as described above, although often made to higher standards of quality with a much higher price, and arrangements for distribution by suppliers of medical equipment.
The purpose is not to cure the condition, but to attain an erection by mechanical and hydrodynamical means even if there is nerve and vascular damage preventing a full natural erection; once the penis is erect, a compression ring is slid on to maintain the erection which will otherwise subside immediately.
A flexible compression ring (or sometimes more than one), commonly called cock rings when used recreationally, must be used. It is fitted on the open end of the cylinder, then an erection is created by pumping. Then the ring is pushed by hand onto the base of the erect penis before releasing the vacuum.
This restricts blood flow out of the now erect penis, enabling the erection to be held even in the presence of problems of the vascular or nervous system which would otherwise lead to immediate loss of erection.
In the best circumstances erections can be maintained for a considerable time, but manufacturers' literature recommends that, for safety, rings should be removed after no more than 30 minutes. Very prolonged use (hours) will cause permanent harm.
A personal lubricant of a type not harmful to the ring material is used, primarily to make a good seal at the base of the pump to prevent air leakage, in the same way as vacuum grease is used with a laboratory vacuum pump,. It also makes it easier to slide the ring off the cylinder, and later to remove the ring.
Pumping must be done very carefully to avoid serious injury: over-enthusiastic pumping can burst blood vessels and form blisters. In some cases the testicles can be unexpectedly pulled into the cylinder, causing discomfort, pain, and possibly injury.
It is also believed that the rim of the cylinder can cut into the skin and over time cause damage to the ligaments surrounding the penis. Impatiently pumping without reading explanatory material can produce too much suction (any pressure lower than 10 torr/1,333 Pa) and cause permanent and irreparable injury.
Attempts at using vacuum cleaner units for this purpose have resulted in extremely severe injuries because the suction is far too powerful.
Pumps used with rings are virtually always effective, as they operate by a simple mechanical process, in cases of ED, even when pharmaceutical methods fail. They may nonetheless be found unsatisfactory due to the inconvenience and similar factors.
The effectiveness of penis pumps for permanent penis enlargement was examined by Kazem, Hosseini and Alizadeh. They studied 37 men with penis length less than 10 cm and found no significant change in penile length after using pumps for six months, although the follow-up have found 30% satisfaction with the method.
A 31-patient study conducted by the Department of Urology at St Peter's Hospitals and the Institute of Urology in London investigating the usefulness of pumps to correct the penile curvature associated with Peyronie's disease found that "There was a clinically and statistically significant improvement in penile length, angle of curvature and pain after 12 weeks of using the vacuum pump".
In this study, subjects with Peyronie's undertook two ten-minute pumping sessions per day for twelve weeks. The additional penis length was an unexpected side effect and not an intended aim of the study, but large enough to be statistically significant.