By Greta Kaul

By the end of next year, about 20 million patients from the nation’s largest health insurer will be able to use their smartphones to consult with doctors, cutting costs and improving access to care, advocates say.

For the same co-payment — or less — than patients are charged for a visit to the doctor, UnitedHealthcare will allow patients to connect within minutes, thanks to a partnership with three telemedicine companies, including San Francisco startup Doctor on Demand, UnitedHealthcare announced Thursday.

“That prevents waiting a couple of days or a couple of weeks to get an appointment — or sitting in an urgent care for a few hours waiting,” said Adam Jackson, a co-founder and the chief executive officer of Doctor on Demand.

The average digital visit lasts 10 minutes, and if the doctor prescribes a medication, the patient can choose a pharmacy through the platform.

It’s the first time Doctor on Demand, which has 1,400 doctors on its platform, is an in-network provider for a health insurance company, Jackson said. Founded in 2012, the company also offers $40 visits to patients, who pay out of pocket. Its team includes daytime TV host Dr. Phil McGraw and his son, Jay McGraw, who are co-founders.

The move by UnitedHealthcare could persuade other insurers to beef up their telemedicine offerings.

Initially, UnitedHealthcare will offer the service to about a million customers, with self-funded employer insurance, a type often found at large companies. The service will be available to most people covered by UnitedHealthcare by the end of 2016, said Karen Scott, a senior director of marketing for product and innovation at UnitedHealthcare.

The Minnetonka, Minn., insurance company said it will also work with telemedicine platforms from Optum, a division of the insurer’s parent company, and American Well, a Boston firm.

Proponents of telemedicine say the technology cuts health care costs and increases access to doctors, especially for those who live far away from clinics.

Telemedicine could cut back office and emergency room visits by 15 percent each, and 37 percent of urgent care visits, according to research by Towers Watson, a professional services firm. Roughly 20 percent of employer plans currently offer telemedicine; more than 70 percent plan to by 2017. Medicare offers it on a limited basis.

“Just about every visit that occurs with Doctor on Demand is a more expensive in-office visit that didn’t happen,” Jackson said. “An urgent care visit is about $250, and an (emergency room) visit for a non-emergency could exceed $1,000.”

Telemedicine companies, including Doctor on Demand and competitors HealthTap and Teledoc, are growing with the help of venture capital funding. Doctor on Demand has raised $24 million from the likes of Shasta Ventures, Google Ventures, Andreessen Horowitz and Richard Branson.

UnitedHealthcare’s decision to expand online offerings was largely driven by patients, Scott said.

“In the last 12 to 18 months, there has been growing interest in the consumer market overall, just a groundswell of interest in being able to access virtual visits,” she said.

There have also been discussions in the medical industry about when it’s appropriate to connect with a doctor online.

Telemedicine has tremendous potential in both preventive care and in the management of chronic disease, which is one of the most expensive and preventable segments of health care spending, said Dr. Seth Bokser, a clinical informaticist and medical director of information technology for UCSF’s Betty Irene Moore Women’s Hospital and Benioff Children’s Hospital.

But it should be used as a supplement to traditional care — not as a substitute, he said.

Though screens bring doctors into the home, they usually aren’t the same as house calls, or office visits, because patients tend to see a doctor they’ve never met before and won’t likely see again.

“The difference between the traditional house call model and many of the telehealth models is that when the doctor made a physical house call, it was usually your doctor, a doctor who knew you and your family,” he said.

While telemedicine works well for certain conditions, the relationship between doctors and patients remains important, Bokser said. When patients trust their doctor, the physician’s reassurance alone can be therapeutic. Amid growing concern about overprescribing drugs, there are fears physicians who don’t know their patients could be wind up unnecessarily prescribing medication to pacify them.

“We all need to go into it with the expectation that some medical conditions are effectively managed on a transactional basis between, for example, a healthy young person with a common ailment and a physician that they haven’t met before,” Bokser said.