Distribution of deaths by unintentional illicit drug overdose in Italy based on periodicity over time, 1984-2000

Aim: To assess whether substance-abusing people have a higher risk of dying from unintentional acute intoxication in specific periods of the year. Methods: A total of 15 792 deaths (males=14 259; females=1533) were ascertained to be due to unintentional acute intoxication by illicit drugs in Italy in the study period (1984-2000). Distribution of deaths by month was studied with circular statistics techniques, based on the maximization of mean vector length (MMVL) method. The Rayleigh test was used to verify the hypothesis of uniform distribution of the events across some relevant periods. Density estimates for significant periodicities were derived with the kernel method for circular variables. Results: Over the study period there was a steady increase in the number of deaths by overdose in Italy, affecting both genders, but more evident among males. The monthly distribution of deaths over the study period followed an uneven trend, with a slightly higher risk in the period between December and January, and in August, in both genders. The monthly distribution of deaths due to unintentional acute intoxication by illicit drugs appears influenced by non-casual oscillations following a thrice-yearly cycle, interlaced with a significantly more evident 6-monthly recurrence. Conclusions: There does seem to be a higher risk of dying from unintentional overdose in specific periods of the year. Better knowledge of the factors affecting the risk of mortality among substance-abusing people - due to changes in environmental or biological rhythms - could allow prediction of negative events, hence their prevention. © 2003 Elsevier Ireland Ltd. All rights reserved.