Four women have come forward after apparently being injected by strangers with drugs during San Fermin on Saturday night and early Sunday morning.
The victims went to the emergency services in Pamplona reporting symptoms of dizziness and feeling as though they were about to lose consciousness.
The first case allegedly took place on Saturday night, with several more reported around 2am.
The four women stated that they felt disoriented after feeling a prick in their arm in what they described as a sensation similar to the feeling of being administered an injection.
Police said they were taking the reports extremely seriously and advised anyone who suspects they are the victim of drugging or who feel dizzy or disoriented to seek medical assistance immediately.
In recent months there have been a number of cases of women reporting they had been drugged intravenously.
In October of last year, a spate of druggings at nightclubs in the UK were reported across several cities, with victims being injected with unknown substances.