The market for drugs to cure erectile dysfunction is being widened to include a tablet that can last up to 36 hours.

Cialis, a new treatment for erectile dysfunction not yet available in the United States, is the product of a joint venture between ICOS Corporation and Eli Lilly and Company.

Erectile dysfunction is the consistent inability to attain and maintain an erection sufficient for sexual intercourse. It affects an estimated 152 million men and their partners worldwide and more than 30 million men in the United States. It is often caused by physical conditions such as heart disease and diabetes.

Carole Copeland, spokeswoman for Eli Lilly and Company, said Cialis is a phosphodiesterase type 5 (PDE5) inhibitor, placing it in the same class of drugs as Levitra and Viagra. The major difference is that its primary ingredient is tadalafil, as opposed to vardenafi or sildenafil.

All three drugs work by inhibiting the PDE5 enzyme, which causes a relaxing of smooth muscle and inflow of blood into penile tissues, producing an erection.

Copeland said the drug's ability to last up to 36 hours and not be affected by food is what helps make it unique, because other drugs do not work as long and can be deterred by high fat meals.

"Men say they have pressure to perform, which puts strain on the man and the relationship," Copeland said. "He's worried he can't respond and the extra planning required by other products adds more pressure to the situation. Cialis relieves some of that pressure."

Copeland said the benefit of the window of opportunity is that it allows men to return to a normal sex life.

Cialis does not give men a continuous 36-hour erection. The drug, like its competitors, only works with sexual stimulation and does not act as an aphrodisiac or increase libido.

The drug is intended for 40- to 70-year-old men with erectile dysfunction and is not a drug for people without a prescription, Copeland said.

"One of the benefits of Cialis is that it forces men to go to their doctor and have a physical, which can help them find other conditions," Copeland said.

Pfizer spokesman Daniel Watts said he thinks it is good to raise awareness of the condition, but from a competitive standpoint thinks Viagra is going to remain the drug of choice.

"It is dependable and (men) have five years of experience with the product (Viagra)," Watts said.

Watts said he thinks the 36-hour time frame of Cialis is unnecessarily long, and is concerned about the effects of men having an extra drug in their system for many hours at a time.

"We found that most people who engage in sex do so in a four-hour time frame, which is when Viagra works," Watts said. "Viagra is the perfect drug for most people."

Watts said despite the hype surrounding erectile dysfunction, it is a medical condition often caused by unavoidable factors. The loss of self esteem that often accompanies erectile dysfunction can cause men to become depressed as well.

David Lawrance, medical director of McKinley Health Center, said in an e-mail that he thinks there are many misunderstandings surrounding the disorder, and there are "sound medical reasons" to prescribe drugs for it.

Lawrance said cases of impotence can be classified into two broad categories: those caused by physiological factors and those caused by psychological factors.

"The changes associated with aging and of progressive vascular disease are examples of the former," Lawrance said. "In the latter category are anxiety, depression, and so on. The medicines affect physiology."

Watts said men often develop psychological symptoms after they realize the physical symptoms, but said the emergence of Viagra and other drugs meant to cure erectile dysfunction have helped to destigmatize the condition.

There are some reported side effects of Cialis, which include mild to moderate headaches and indigestion, Copeland said.

She said Cialis is in the process of being reviewed by the FDA. It is sold in 45 countries and Copeland said she hopes it will be released in the United States by the end of next month.