Seasonal variation in suicides and in deaths by unintentional illicit acute drug intoxications.

In order to assess whether substance-abusing people show a higher risk of dying from unintentional acute intoxication in specific periods of the year, and whether this unevenly distributed risk is subject to the same seasonal influences observed in suicide cases, we studied a total of 15,792 drug-related deaths (males=14,259; females=1533) and 20,332 deaths by suicide (males=15,571; females=4761), ascertained in Italy among 15-44-year-old people from 1984 to 2000. Distribution of deaths by month was studied with circular statistic techniques, based on the maximization of mean vector length method and the Rayleigh test. The monthly distribution of deaths over the study period followed an uneven trend that clearly differed, however, between the two time-series characterizing suicides and drug-related deaths. A clear seasonal variation was found for suicides in both genders, recurring on a significant circannual cycle. No circannual cycle was observed in the monthly distribution of drug-related deaths, which seem to be influenced by non-casual oscillations following a thrice-yearly cycle, linked to a significantly more evident 6-month recurrence. Some periods of the year, however, are clearly and consistently linked to an enhanced risk of dying by unintentional, acute intoxication by illicit drugs.