According to two medical professionals from the Department of Public Health and General Practice, University of Otago, Christchurch, New Zealand and writing in theBMJ

Industry funded patient information and the slippery slope to New Zealand

https://pmc.ncbi.nlm.nih.gov/articles/PMC2001072/

  In New Zealand and the US, the only two developed countries that allow direct to consumer advertising of prescription medicines, opposition has grown steadily from both the public and doctors. New Zealand's health system is much closer to those in Europe than the US system. So what can we learn from its experience?

  Rise of advertising

  Unlike most other developed countries, New Zealand never enacted pre-emptive legislation to prevent direct to consumer advertising.

  The adverts started appearing in the early 1990s, and steadily increased.

  But the US Food and Drugs Administration relaxation of regulatory requirements for broadcast advertising in 1997, unleashed an explosion in both the US and New Zealand.

  Last year drug companies spent over $5bn (£2.5bn; €3.6bn) on direct to consumer advertising in the US and tens of millions of dollars in New Zealand.

It was as much absence of barriers to entry as it was a strong armed entry.