A ransomware attack on a US health care tech company has caused billing delays and security concerns, impacting the prescription market and leading to outages affecting doctors and pharmacies. The cyberattack on Change Healthcare, a UnitedHealth unit, has disrupted prescription processing for millions of Americans, with the outage expected to last weeks. The attack has forced some pharmacies to make difficult decisions, risking non-payment or denying medication. The incident, described as the most serious against a US health care organization, has left small practices and health-care providers facing financial pressures as crucial reimbursement systems remain down for the ninth day.