The latest study, published in the medical journal Menopause, is based on surveys of more than 24,000 women taking part in an ovarian cancer screening study in Britain. The women, aged 50 to 74, answered multiple-choice health questionnaires about their sex lives at the start of the study. But the survey data are unique because about 4,500 of the women also left written comments, giving researchers a trove of new insights about women’s sex lives.

Over all, 78 percent of the women surveyed said they had an intimate partner, but fewer than half the women (49.2 percent) said they had active sex lives. The women’s written answers about why they stopped having sex revealed the pain and sadness behind the percentages.

The main reason was losing a partner to death or divorce, which was cited by 37 percent of the women. (Women who were not having sex cited multiple reasons for the decline, which is why the percentages exceed 100.)

‘‘I have been a widow for 17 years. My husband was my childhood sweetheart, there will never be anyone else.’’ (Age 72)

Some women said life was too complicated to make time for sex — 8 percent said their partner was too tired for sex, and 9 percent of women said they were also too tired for sex.

“I feel my role in life at present is to bring up my 12-year-old son; relationships come second.” (Age 50) “Caring for older parents at the present. Lack of energy and worrying about them causes a reduction in sexual activity.” (Age 53) “Husband busy with work. I’m busy with two children. Both collapse into bed at the end of the day.” (Age 50)

A husband with serious health issues was another common theme. About one in four women (23 percent) said the lack of sex was because of their partner’s physical problems, and 11 percent of women blamed their own physical problems.

“He does not maintain erection strong enough for penetration (after prostate surgery and diabetes). My sexual activity is limited by what my husband’s health is.” (Age 59) “My husband had a stroke which left him paralyzed. Sexual relations are too difficult. I remain with him as a caregiver and companion.” (Age 52) “My husband has had a heart attack — his medication leaves side effects, which makes sex very difficult, which has saddened us.” (Age 62)

Others cited mental health and addiction issues as the reason for lack of sex.

“He drinks approximately 1 to 1.5 bottles of whiskey a day. Sex is once or twice a year.” (Age 56) “My husband suffers from anxiety and depression and this has an effect on our relationship and my sleeping.” (Age 53) “I take an antidepressant which blunts desire for sex.” (Age 59)

About 30 percent of women said their sex lives had halted because they had “no interest.”

“Have lost all interest and feel guilty, and that makes me avoid any mention of it at all.” (Age 53) “Several symptoms of the menopause have affected my desire for sex, which I find disappointing because I wish I had the same desire as I had in recent years.” (Age 58) “I find it uncomfortable and sometimes painful. I use vaginal gels but doesn’t help much, so do not have sex these last months.” (Age 54) “I love my partner very much, this problem upsets me. However if I didn’t have a partner (for sex) I wouldn’t miss it — it’s very hard to desire something you don’t want. I feel sad when I think of how we used to be. He is very understanding.” (Age 54)

And 21 percent of women said their partners had lost interest in sex.

“Only [have sex] twice a year maybe. My partner has lost his libido and never thinks of it, although he loves me and worries about it.” (Age 60)

While most of the written comments were about problems with sex, a few women left more hopeful messages.

“As I have a new partner since one year, I find my sexual life has never been better and it is certainly very frequent. Very much the reason for my happiness, contentment and well-being.” (Age 59) Sex happens “less often than when younger. We both get tired, but when we do it, it’s good.” (Age 64)

The data and comments were analyzed by Dr. Helena Harder, a research fellow at Brighton and Sussex Medical School, and colleagues. Dr. Harder said the comments show that doctors need to have more frequent conversations with women about sex.

“Women say that they are sorry that things have changed. They wish it was different,” says Dr. Harder. “But in general, it’s not being brought up in discussions. Patients need reassurance that it’s O.K. to discuss sex and ask questions. If you do that, it’s probably a good step toward making changes.”

Dr. Faubion, who is also medical director for the North American Menopause Society, notes that treatments are available to help women with vaginal dryness and painful sex. In addition, two libido drugs have been approved to help increase female desire. One is a pill and the other, an injectable, should be available this fall, although both drugs have drawbacks, including cost, limits on when they can be used and side effects, so they aren’t an option for every woman, she said.