Drugs that treat incontinence caused by an overactive bladder offer modest benefits to some women, and they often come with significant side effects, a new review of research shows.

The government-funded review compared the benefits and side effects of several drugs: darifenacin (Enablex), fesoterodine (Toviaz), oxybutynin (Ditropan, Oxytrol), solifenacin (Vesicare), tolterodine (Detrol), and trospium (Sanctura).

Each drug is different. But they all work by relaxing the bladder muscle, reducing spasms that can cause urgency and leakage.

The new review is based on data from 94 studies that compared at least one of the medications to a placebo pill.

As a group, the drugs all helped women achieve continence more frequently than placebo pills. The researchers estimate that for every 1,000 women who were treated with the drugs, less than 200, or 20%, achieved continence on the medications.

But side effects were also common.

"One of the things we report is that 50% of the women in the study stopped treatment with the drugs within one year," says researcher Robert L. Kane, MD, a professor of long-term care and aging at the University of Minnesota?s School of Public Health in Minneapolis. "Basically, it's because of side effects."

The most commonly reported side effect of the drugs was dry mouth. Other common side effects included constipation, dry skin, dry eyes, and upset stomach.

People were most likely to quit the drug oxybutynin because of side effects.

Side effects were least often reported by people taking the drug solifenacin. In long-term follow-up, one drug, tolterodine was strongly linked to a greater risk of hallucinations.

And older people who used incontinence medications in combination with other drugs, like antihistamines, were more likely to experience abnormal heart rhythms problems and sudden death.

Those side effects are not new, however. They had been noted in previous studies.

OAB: Helping a Confused Bladder
Weighing Benefits and Risks

The review does not make recommendations about the use of these drugs.

"It turns out that it's as important to sit down with your doctor and look at the side effect pattern of the pills as it is the direct effects," Kane says, "because for the most part, a number of them basically have about the same effectiveness, but they have different patterns of side effects."

The new study is published in the Annals of Internal Medicine. It was funded by the federal Agency for Healthcare Research and Quality.

"This is a very important and well-done study," says Patricia Goode, MD, a professor of medicine in the Center for Aging at the University of Alabama at Birmingham.

Goode says in an email that studies comparing the benefits and drawbacks of similar drugs are rare, "so this type of study, providing all of the best effectiveness data in one place, will be very useful."

But she says the research may also be a little misleading since it masks the small degrees of improvement many women see on the drugs.

"Improvement alone can improve quality of life -- for example, going from diapers to pads, or no???longer having accidents that overflow pads and cause embarrassment," she says. "Both frequency and volume decreases are important."

But she says that for women who are able, Kegel exercises and lifestyle changes that include drinking less caffeine and watching fluid intake are likely to have bigger impacts on the condition.

"They have very virtually no side effects and can be very effective," she says.