Officials said UCSF is treating two patients who overdosed from an unknown substance. They are both stable and under observation by the hospital.
VIDEO: Bigger crowds expected at SF's 4/20 celebration now that cannabis is legal
At this time, officials have not said what condition the other patients are in.
Friends of a man that collapsed told ABC7 News that he was near the stage when it happened, but was awake when paramedics got to him.
Officials said at least 20,000 people were at Golden Gate Park Friday enjoying 4/20. Last year, about 10,000 people attended the event.
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.@SFPD looking into fentanyl as possible cause of several medical emergencies during #420day event at #HippieHill in #SF. 12 people transported to hospitals. Several people given Narcan to counter the OD. #abc7now pic.twitter.com/dBIp26y1D7— Katie Utehs (@KatieUtehs) April 21, 2018