Few men's penises are completely straight when they are erect. It is common for an erect penis to have a slight curve in it. The curve can be up or down, or to the left or right. A curve of up to 30° is considered normal, and medical treatment is usually not needed unless the angle of curvature is more than 45°. A curved penis does not usually cause a man any problems when he is having sex. However, if the curvature of a penis changes, this could be caused by an illness called Peyronie's disease (see below). An erection that pokes through clothing is sometimes called a "manbulge".
A survey of 300 men over the age of 18 carried out between 11 March and 17 March 2001 in Cancún, Mexico, found that among those men the average erect human penis was about 14.928 centimetres (5.877 inches) long, and that most of them had an erect penis length of between 14 and 16 centimetres (5.5 and 6.3 inches). The typical girth or circumference (the length around the outside of the penis) was about 12.63 centimetres (4.972 inches), and most erect penises had a girth of between 12 and 13 centimetres (4.7 and 5.1 inches). As this survey was only done in one city, its findings may not apply to men from other countries.
The size of an erect penis generally does not change after puberty, the time when a boy becomes sexually mature. Some people have surgery to make their penises larger. However, a study that came out in 2006 found that only 35% of men who had gone for such surgery were happy with the results.
An erection of the penis can also happen by itself when a man is asleep, often when he is dreaming. The scientific term for this is "nocturnal penile tumescence" (which means "swelling of the penis at night"). It is also common for a man to wake up after a night's sleep to find that he has an erection. This is often caused by his bladder being full of urine, which presses on the tissues in the penis. Such an erection is sometimes informally called a "morning glory" or "morning wood".
Most boy teenagers have unexpected erections. Sometimes adult men have unexpected erections. Unexpected erections are involuntary and are normal. People call this a "spontaneous" (sudden or surprise) erection. Sometimes it happens in school. Sometimes it happens walking. Sometimes it happens taking clothes off for a shower or bath or swimming. This is normal. Such erections can be embarrassing if they happen in public. Erections can occur unexpectedly at any time of day.
A man who often has problems having or keeping an erection of his penis is said to have erectile dysfunction. Erectile dysfunction is also known as ED, impotence or male impotence. The study of erectile dysfunction comes within andrology, the field of science dealing with men's health.
ED is not uncommon. For example, about 10% of all men in the United Kingdom will have ED happening to them regularly at some time in their lives. It can be due to both physiological and psychological reasons. Physiological reasons are those dealing with how the body works. Psychological reasons are those involving human behaviour and how people think.
These are some common physiological reasons for ED:
ED can also happen for psychological reasons. For example, a man can find it difficult to have an erection:
Having erections is seen by many people as a very important part of being a man. Therefore, a man who has ED often feels that he is not manly enough and not successful in life. This can lead to him feeling ashamed and not as good as other men. The matter is made worse because men often do not like talking about their personal problems with other people, so they keep their unhappy feelings to themselves. However, talking to a doctor or psychiatrist (a doctor who knows a lot about mental problems) is helpful, as he or she may be able to suggest ways of dealing with the problem.
How ED may be treated depends on the cause. If it is caused by a disease, sometimes treating the disease will reduce the problem. For example, if a man is unable to have erections because his body does not make enough testosterone, he can be given pills to take which help his body to do so. If the blood vessels or nerves in the corpora cavernosa penis are damaged, a full cure may not be possible. However, there are some temporary treatments that allow a man to have an erection for long enough to have sex:
If a man's erection lasts for more than four hours even though he is no longer sexually excited, this is a medical problem called priapism (). The man must go to a doctor or hospital quickly if it happens. The condition is named after the Greek god Priapus, whose penis was believed to always be erect (see below). Priapism is painful, and is a medical emergency because the blood stuck in the penis can become thick and form clots. This can damage the blood vessels of the penis and lead to erectile dysfunction (ED). In serious cases the tissues of the penis can die, and the penis may have to be amputated (cut off) so that the tissue death does not spread to the rest of the body.
It is not really known what causes priapism. Men who suffer from priapism sometimes have blood diseases like leukemia (a type of cancer of the blood; in the United Kingdom and some other countries it is spelled "leukaemia") or sickle-cell disease (which makes the blood cells crescent-shaped). Problems with the nervous system such as injury to the spinal cord or abnormal growths on it can also lead to priapism. Certain medicines can make priapism happen, including some used to treat ED such as alprostadil. A 2008 study suggests that too much of the chemical adenosine in the body may cause priapism since it makes blood vessels widen which lets more blood flow into the penis.
When a man with priapism is brought to the emergency department of a hospital, a doctor will usually try to remove blood from the corpora cavernosa penis by aspirating it (sucking it out) using a syringe. If this is not enough, a chemical called phenylephrine (/"feen-il-eff-rin"/) may be injected into the penis to try and make the blood vessels become narrower. If this also does not work, the doctor will make a shunt (a hole or passage) between the corpora cavernosa penis and corpus spongiosum penis. This allows blood to flow away from the corpora cavernosa penis into the corpus spongiosum penis. For men who have priapism again and again, it has been found that medicines such as sildenafil (Viagra) and tadalafil (Cialis) may help.
When an erect penis is shown in art, it is often called a phallus. Erotic (sexually exciting) art has shown phalluses for a very long time. Pictures of men with erections appear on ancient objects and in paintings.
The erect penis was also a symbol or sign of health and fertility (the ability to give life). The Hohle Fels phallus was found in a cave in Germany. It is a piece of stone carved to look like a penis that archaeologists believe is about 28,000 years old. (Archaeologists are scientists who study the past by looking for things that people have left behind.) From the fourth millennium B.C. (4000–3001 B.C.), Ancient Egyptians worshipped Min as the god of reproduction and the maker of all things. Min was shown in statues and on wall carvings as having an erect penis.
The Ancient Greeks believed in a god called Priapus who had a very large penis that was always erect. He was thought to protect livestock (animals kept by humans for food, milk, leather or wool), fruit plants and gardens, and men's sex organs. He was also seen as able to chase away evil, and as a protector of sailors, fishermen and others needing good luck.
Long after the Roman Empire ended and many people became Christian, Priapus was still seen as a symbol of health and fertility. A 13th-century book called the "Lanercost Chronicle", which is a history of northern England and Scotland, talks about someone putting up a statue of Priapus to try and stop a disease from spreading among cattle.