Carolyn Marrow, 75, suffers from a pinched nerve in her back that affects her walking, and so at least once a month she visits her doctor several miles away from her senior living community in Redwood City, California.

Marrow uses her community’s shuttle service when she can, but it often stops running before many of her doctor’s appointments. When that happens, Marrow uses Lyft. Her assisted living community, Brookdale Redwood City, formed a partnership with the ride-hailing company in 2016.

Residents book a Lyft via a concierge in their community, and the rides are billed to residents’ rooms at the end of the month. About 35 people use the service each month, Brookdale said.

“It’s like going in a private car, and I don’t have to worry about if I’m going to make my appointment,” said Marrow, who spends $20 to $25 a month on the rides. Before the Lyft service existed, she told CNBC, she had considered skipping her appointments.

Lyft and rival Uber are carving out a niche in the nonemergency medical transportation market, attempting to address the need for adequate transportation to medical appointments. Both have dozens of partnerships like the one at Brookdale.

The companies charge about the same as a normal pickup for the service but there is an added convenience for those who may not be tech savvy enough or have access to a mobile app.

According to a 2006 National Institutes of Health study, about 3.6 million Americans miss at least one medical appointment or delay medical care each year due to a lack of reliable transportation. Additionally, Health Outreach Partners found in its 2017 Rides to Wellness report that 25 percent of missed appointments in the U.S. are caused by transportation barriers.

Rich Sampson, communications director at Community Transportation Association of America, told CNBC it may be too early to gauge the impact transportation partnerships are having on medical care.

“Meaningful, new data isn’t available yet, and the approaches taken in each partnership vary,” said Sampson, whose organization tracks trends in mobility.

However, a study published in February in the peer-reviewed JAMA Internal Medicine found giving low-income people, insured by Medicaid, free use of ride-hailing services for medical appointments doesn’t make them any less likely to become no-shows.

The study included nearly 800 Philadelphians who were patients with Medicaid at one of two Penn Medicine primary care practices.

Krisda Chaiyachati, lead author of the study, told CNBC that the population of patients who didn’t use the free ride-hailing service could have played a role in the study’s results.

He said low-income patients often have hectic lives and may negotiate whether an appointment is important or not. “If it’s high blood pressure, you might think it’s OK to miss an appointment. But if it’s cancer, then you might go,” said Chaiyachati, a medical director for Penn Medicine.

Despite the results, Chaiyachati expects there could be an overall trend of more patients using ride-hailing services to make appointments.

Alexandra King, program associate at the Community Transportation Association of America, told CNBC that study definitely “shook up” the conversation around the merits of ride-hailing services providing medical transportation. But she also said there were some limitations regarding the study, including that it only looked at primary care practices.

Additionally, it was limited to people insured by Medicaid and did not involve a variety of locations, King said.

“The study is a very relevant [one]. But [medical transportation] is such a complex issue, and one study can’t get an overall view of the entire thing,” she added.

“We can confidently state that, overall, access to improved transportation options does reduce the number of missed appointments,” Sampson said.

Uber and Lyft are moving ahead, striking dozens of partnerships with health-care organizations and hospitals.

In March, Uber launched Uber Health, which allows health-care workers to book rides for patients on demand or for future doctors’ appointments. Patients receive a text or call at the time the ride is booked and no additional action is required on behalf of the health-care worker.

Lauren Steingold, head of strategy at Uber Health, told CNBC that since the beginning of the new business line, hospitals and health-care organizations have told the company the number of missed appointments by patients has dropped.

Steingold declined to provide specific numbers because she said the business is currently in beta testing and Uber does not typically share such data. (CNBC reached out several hospitals in partnership with Uber, and they declined to comment as well.)

Likewise, David Baga, chief business officer at Lyft, also declined to provide specific numbers for its business. However, he told CNBC the company’s medical transportation business serves “tens of millions” of patients.

Baga, who leads the Lyft Mobility Solutions enterprise team, said medical transportation is the largest portion of the unit, and he expects it could be a billion-dollar enterprise someday.

Brookdale resident Marrow, who is unmarried and whose daughter lives in Oregon, said she expects to continue to use the Lyft service in her community for the foreseeable future. She added that the service is just too convenient for her and she’s had positive experiences.

“I think it’s great,” Marrow said. “The drivers, the ones I’ve had, are young and very friendly and talkative, and you feel like you get to know them by the time I finish my ride.”