"Between 2012 and 2017, 22,000 Venezuelan doctors migrated" -- 55 percent of the total 39,000 doctors registered by the PanAmerican Health Organization in 2014, the report said.
Some 6,000 nurses -- nearly a quarter of Venezuela's total -- also left the country during the same period.
Nearly two million people have emigrated since 2015, the UN said, adding that around 5,000 people are leaving the country daily.
The NGO report -- "The right to health, the complex humanitarian emergency in Venezuela" -- was endorsed by 12 non-governmental organizations for patient rights.
Out of Venezuela's population of 30 million, some "18.7 million have no guarantees of access to diagnosis or treatment," it said.
It adds up to "a humanitarian emergency," the NGOs said.
The health crisis is exacerbated by an acute shortage of medicines in pharmacies and hospitals across the country, according to trade associations.
The United Nations last week announced a $32 million donation to reduce maternal and infant mortality and to protect children, in addition to donating 30 tons of health supplies.
With hyperinflation expected to reach 1.3 million percent in 2018, the few medicines available are unattainable for most of the population, as is private health care.
Oil income-dependent Venezuela's economic woes began in 2014 with the crash in the price of crude.
President Nicolas Maduro's government has since been slapped with a range of sanctions over its crackdown on the opposition and civil society critics.
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