Anthem Blue Cross told Lauerman the drug wasn't necessary. She eventually got it after an infection landed her in the hospital, but that led to another problem: She ended up missing several chemo sessions.

"The insurance has been great until I got to a point that I really needed something for survival," Lauerman said.

Doctors say they worry about the growing influence insurers have over patient care. Some are finding that they need more approvals from insurance companies for routine things like medical scans or some prescriptions, which can postpone care for a few days or even weeks.

Insurers say advances in medical care are prompting them to review more cases before deciding on coverage. They say the checks are not meant to delay or stifle care, and they see them as a way to talk to doctors about the best approach and to guard against unnecessary treatment.

"It's not the end of the conversation," said Kristine Grow, a spokeswoman for the insurer trade group America's Health Insurance Plans.

Stuck in the middle are patients who may wind up with breaks in treatment. Those interruptions can stir anxiety and, in some cases, influence the success of their care.