The researchers at the Australian National Drug and Alcohol Research Center and the National Drug Research Institute say increased potency of marijuana has been observed in some countries but there is enormous variation between samples -- meaning that marijuana users may be exposed to greater variation in the strength of the cannabis they use in a single year than over years or decades.

Marijuana samples tested in the United States, the Netherlands, the United Kingdom and Italy have shown increases in potency over the last decade but not in other European countries or in New Zealand.

The study, published in the journal Addiction, said studies say that some marijuana smokers, when faced with a "strong" product, act like tobacco smokers and adjust their dose by increasing the interval between puffs, or holding smoke in their lungs for a shorter period of time. This behavior may reduce possible harm caused by increased potency.

The authors also discuss the health risks of contaminants including naturally occurring ones like fungi, growth enhancers and pesticides as well as substances added to "bulk up" the weight of marijuana.