An Auckland man has admitted to a charge of attempting to stupefy after spiking McDonald's ice cream sundaes at a west Auckland store.

The 22-year-old Rodney man pleaded guilty at the Auckland District Court in front Judge Nevin Dawson, who continued his interim name suppression.

The man had been charged with one count of attempting to stupefy after prescription medication was found in several sundaes bought from Te Atatu McDonald's in May last year.

The man was not an employee of McDonald's, police confirmed at the time.

The people said at the time they took the ice creams home and ate them, discovering halfway through that there was prescription medication in the desserts.

"We ate them at home, and we all found traces and whole pills of prescription medicine. My sister ingested one. I found one in my mouth and spat it out," one said on social media.

"The police are involved. We got off so lucky. Had a child ingested it. There would be more of an issue. I don't know who would do such a thing but its disgusting [sic]!!"

Speaking to the Herald, one of the people who ate the ice creams explained it all started when her sister, while eating her sundae, complained about "an extremely funny taste".

"My partner and my sister went to leave when I found one [pill] in mine. We went straight to hospital they said it was someone's medication. But it's not enough to do any damage so suggested keeping going with police."

A McDonald's spokesperson last year said the investigation had cleared all staff of any wrongdoing.

"It is disappointing that somebody would make an allegation of this nature, and that the subsequent publicity on news and social media unfairly damaged our reputation, took up a significant amount of time for our people, and resulted in a loss of sales."

The man is due to be sentenced on June 20.