Side-effects can include headaches, blurred vision and upset stomach
A cream allowing erectile dysfunction drugs to be applied directly to the skin could one day make them safer to use, say New York scientists.
Studies in rats suggest that Viagra, Levitra and Cialis could pass through the skin in tiny capsules, they say.
The research, published in the Journal of Sexual Medicine, could mean fewer side-effects, and even significantly speed up the drug's action.
However, it could be a decade before creams are fully ready for use.
The arrival of erectile dysfunction treatments in tablet form has been one of the success stories of the modern pharmaceutical industry, with some estimates suggesting that tens of millions of men worldwide have used them.
However, although they have worked for many men, they also carry the risk of side-effects such as headaches, blurred vision or upset stomachs.
In addition, men with severe heart problems, or who have just suffered a stroke, are advised to avoid the tablets altogether or use them with extreme caution.
For many, this could be solved by the development of the cream, with would confine more of the active ingredients of the drug to a single area of the body, rather than circulating them widely.
The research team at the Albert Einstein College of Medicine at Yeshiva University, in New York, used nanoparticles, each much smaller than a grain of pollen, and found a way to encapsulate particles of the drug inside.
Their early tests involved just a few rats bred to have erectile dysfunction later in life.
Of these, 11 were treated with nanoparticles containing Cialis, a newer erectile dysfunction drug called sialorphin, and nitric oxide, a chemical also needed to widen blood vessels and produce an erection, often reduced in men with diabetes.
All of the rats showed improvement, unlike seven rats given empty nanoparticles instead.
Dr Kelvin Davies, one of the researchers, said: "The response time to the nanoparticles was very short, just a few minutes, which is basically what people want in an erectile dysfunction medication.
"In both rats and humans, it can take 30 minutes to one hour for oral erectile dysfunction medications to take effect."
The researchers found no signs of local inflammation or damage caused by the nanoparticles, and no evidence of wider side-effects.
Clinical studies in humans could begin in a few years if animal studies continued to suggest the treatment was safe, they said.
However, finally getting the drug approved for widescale use could take 10 years or more, they said.