US scientists say they are closer to creating a gene therapy treatment for erectile dysfunction.
The gene therapy work is moving from the lab to the clinic
Human and animal trials suggest this could offer an alternative to current treatments for some patients, the American Society of Gene Therapy heard.
Researchers say gene therapy could be effective far longer than pills used just before sex, improving spontaneity.
The human trials involved injections into the penis and some experts queried whether men would choose this option.
Erectile dysfunction (ED), often referred to as impotence, is thought to affect about one in every 10 men in the UK.
ED collectively means an inability to get a good enough erection to achieve satisfactory intercourse, but it varies in severity.
There are many causes, and many effective treatments, including drugs like the three licensed in the UK - Viagra, Cialis and Levitra.
But not all types of ED respond well to medication - for example, ED caused by nerve damage following prostate cancer surgery.
US researchers are hopeful that for these patients, and those who experience side-effects with medication, gene therapy may be a good alternative.
A University of Pittsburgh team, led by Dr Joseph Glorioso, tested an experimental gene therapy in rats with ED caused by nerve damage.
The gene therapy used comprised the herpes simplex virus as a carrier and either a gene called GDNF, or one called neurturin, which both help promote nerve growth.
Rats treated with the gene therapy showed significant recovery and were able to regain normal penile function after four weeks.
Meanwhile, scientists at Wake Forest University, with the help of Dr Arnold Melman from the Albert Einstein College of Medicine, have been conducting the first human study of gene therapy for ED.
Tests on 11 men with ED showed promising results. The treatment was well-tolerated, with few side effects, despite its delivery via an injection into the penis.
Although the primary goal of the study was to determine its safety, it also showed the therapy improved erectile function in some of the men.
Dr Melman said: "This is an exciting field of research because current treatments for men with erectile dysfunction, whether pills or minimally invasive therapies, must be used 'on demand', thereby reducing the spontaneity of the sexual act."
The Wake Forest therapy works by inserting small pieces of DNA into cells to trigger the production of proteins which, in turn, help smooth muscle cells relax.
Relaxing the smooth muscle in the penis allows it to fill with blood and achieve an erection.
Dr Geoff Hackett, president of the British Society for Sexual Medicine, said the gene therapy might be appealing to some men for whom other treatments had failed, but predicted many men would be reluctant to have a shot in the penis.
He stressed that many men with ED also had underlying medical disease, such as high blood pressure, raised cholesterol or diabetes, and that this should also be treated.
And there is a good argument for treating ED as a chronic disease too, he said.
"Maybe men with ED should be taking a pill for it every day. If you take smaller amounts regularly, the side-effects are less," he explained.