Viagra manufacturer Pfizer says more than 15 million people around the world have taken the drug since it was released in 1998, despite a price which is now around $100 for a packet of four pills. Each pill can have an effect for up to 12 hours.
Lead researcher Dr Annie Potts, a research fellow in gender studies at Canterbury University at the time, said about a quarter of the women came forward because they wanted to talk about the detrimental effects of the drug.
The other three-quarters, who often said the drug had allowed them to have sex again, included many who personally "don't necessarily want sex as much as they are getting now".
"Even women who had a generally positive narrative about Viagra explained that a 'downside' or 'drawback' could be an undesired increased frequency of intercourse, because of pressures coming from the desire not to waste a tablet," the study says.
A 51-year-old woman told the researchers that she sometimes felt pressure to have sex both at night and the next morning so that her husband could "get in two for the price of one".
A 48-year-old said: "It had such a powerful effect that ... this made sex inevitable. Sometimes there was no discussion about whether ... the sex act was going to take place, so it would be ... 'I've taken the pill, okay, let's go.' "
She commented: "It took away the whole notion of any kind of ... spontaneity or you know, the reason for the actual bonking. I mean, you know, you like to think it's an act of love, rather than just lust."
Some older women were in pain during sex because of post-menopausal vaginal dryness, even when they used lubricants.
Another 51-year-old said she consented to sex even when she was in pain because she had heard that older men needed to keep having it or they might "lose the art".
Some feared that Viagra would drive their partners to other women. Five of the 33 men confirmed that they had been unfaithful since taking the drug.
The study concluded that women as well as men should take part in doctors' consultations and decisions about erectile problems.
"Men talked about how easy it was to get the drug. Even men who were not experiencing erectile problems found it was easy to go in and get the drug," Dr Potts said.
A 60-year-old woman told the researchers: "We weren't interviewed together ... because they seem to have this idea that this is a man's problem. But it's not a man's problem, it's a couple's problem, and how the woman feels about it should come into it too."
Other women complained about doctors' "clinical coldness". But the head of the Medical Association's General Practitioners' Council, Hawkes Bay GP Peter Foley, said that when male patients asked for Viagra, doctors could not force them to talk to their partners first.
"Most New Zealand doctors would be prescribing Viagra as part of a therapy for a relationship problem," he said. "They would not be prescribing it in isolation without dealing with the other issues in the sexual relationship."
Dr Chris Paltridge of the NZ Men's Clinic said: "I think conversation between the partners is important, and we encourage it, but you can't tell a guy, 'No, go home and grab your wife and come back again.' It's hard enough to get them in there in the first place."
The first report on the study by Dr Potts and three collaborators was published in the British journal Sociology of Health and Illness. Further reports on the experiences of men and other issues are yet to be published.
Annie Potts, Nicola Gavey, Victoria Grace, Tiina Vares:
The Downside of Viagra: Women's Experiences and Concerns
[PDF]
Herald Feature: Health
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