The closure of head shops in Ireland in the summer of 2010 led to a decline in the incidence of drug-related emergency hospital admissions, a new study has found.

Head shops had become increasingly popular from late 2009, particularly among younger adults, due to their sale of potent drugs such as mephedrone and synthetic cannabinoids.

The shops were able to sell these products legally as these new psychoactive substances (NPS) were not specifically mentioned in Ireland's Misuse of Drugs Act, which makes drugs such as cocaine and cannabis illegal.

However in 2010, the sale of these NPSs was made a criminal offence and this essentially led to the closure of head shops nationwide.

Researchers in Trinity College Dublin (TCD) decided to look at the impact this had on emergency hospital admissions. They assessed the pattern of drug-related hospital admissions before, during and after the arrival and departure of these shops in Ireland, focusing on the period 2008 - 2012.

According to the findings, the first eight months of 2010 was the period of greatest activity for these shops, before they disappeared in late August. An analysis found that the rate of drug-related emergency hospital admissions during those first eight months of 2010 was 9% higher than the same period in 2008.

However, the rate of emergency admissions was almost one-third lower in 2012, two years after the shops had closed.

A statistical analysis of the trends estimated that the downward trend in emergency admissions began in June 2010 - a month after the passage of legislation which had criminalised possession and sale of most NPSs, resulting in the immediate closure of half of head shops.

The study specifically looked at admissions into general hospitals via the Emergency Department (ED). It focused on young adults, aged 15 to 34, as they were known to be the heaviest users of the substances that were sold in these shops.

While it was clear in 2010 that some people who used NPSs were experiencing related health problems, it was unclear whether these substances were more harmful than other traditional drugs such as cocaine, cannabis or heroin.

It was also suggested that the health problems related to drug use might deteriorate if head shops were closed, due to a combination of people sourcing these substances from a more dangerous black market and/or returning to use more dangerous older drugs.

However, the findings of this study indicate that such concerns were unfounded. Once the source of supply was removed, health problems reverted back towards their baseline.

"If you look beyond drug policy to alcohol or cigarette policy, these findings should not really come as a surprise. When substance use is normalised and made more accessible, harms escalate. When access is reduced, harms reduce," commented Dr Bobby Smyth, clinical senior lecturer in TCD's Department of Public Health and Primary Care.

The study was a collaboration between TCD's Department of Public Health and Primary Care, the HSE's Health Intelligence Unit and University College Dublin. Details of the findings are published in the European Journal of Public Health.