The tea leaves were purchased at the Sun Wing Wo Trading Company in San Francisco's Chinatown.
Investigators say in separate incidents in February and March, a woman in her 50s and a man in his 30s became critically ill within an hour of drinking tea made from leaves supplied by the same herbalist in San Francisco.
Both victims quickly developed weakness, and then life-threatening abnormal heart rhythms, requiring resuscitation and intensive hospital care.
A plant-based toxin, Aconite, was found in lab tests of the patients and the tea samples they provided.
Residents are being warned if they have any of the following symptoms to seek medical attention:
Numbness or tingling of the face, mouth or limbs
Weakness in the limbs
Dangerously low blood pressure
Slow or fast heartbeat
Irregular heartbeats that can lead to sudden death