The Green Party is calling for the urgent approval of an alternative medication for sufferers of the metabolic disorder hypothyroidism.
The Centre for Adverse Reactions Monitoring says it has received more than 800 reports of health problems in people taking Eltroxin, the only subsided medicine in New Zealand for people with underactive thryoid glands.
Manufacturer GlaxoSmithKline recently changed the drug's formulation, and patients who have since become ill have complained they were not told of the change.
The medicines safety regulator, Medsafe, says a reassessment of the drug prompted by customer complaints found it to be safe.
But the Green's health spokesperson, Sue Kedgley, says she continues to receive floods of emails from people who have become severely ill.
The Centre's director, Michael Tatley, says the number of health problems related to the drug surpasses that of any other changed medication.
Dr Tatley says the system of reporting adverse reactions is voluntary so there's the potential for significant under-reporting of Eltroxin-related problems.