Blake McKinney is a doctor with medical licenses in 49 states and a 50th likely on the way. That would put him into a small but growing group of physicians who see a big future online.

Telemedicine, which involves medical consults via the web and smartphone apps, requires that doctors have licenses to treat patients wherever they are, not just in a single location. With licenses across the country, McKinney can be at home in Denver, but treat a patient in Rhode Island or Delaware or anywhere else in the U.S.

The telemedicine market has been around for more than a decade, but it's tipping into the mainstream as regulations increasingly shift to favor its adoption. The market is expected to hit $130 billion by 2025, according to a recent report, as more insurance plans start covering the cost of virtual visits.

Venture-backed start-ups are now prescribing everything online, whether it's flu medication, hair loss treatments or birth control. The companies all either employ doctors, contract with a physician network or do a bit of both.

Telemedicine start-ups can either hire a lot of doctors with one or two licenses or retain a smaller team of people who can work across many states. McKinney's company CirrusMD, a chat-first virtual primary care provider, chose the latter approach, as did virtual care start-up Lemonaid, which is based in San Francisco and has three doctors on staff, each with 50 licenses.

"Licensing physicians in multiple states is incredibly beneficial for patients," said Zachariah Reitano, the CEO of Ro, an online pharmacy and telemedicine company that prescribes erectile dysfunction medication and hair loss treatments. "It enables a physician to treat patients in care deserts and increases access to care where there aren't enough providers available."