The following paragraph was taken from page 182 of Cannabis: A Complete Guide by Ernest Small.
WHY ELIMINATING TERPENES BY IRRADIATING MEDICAL MARIJUANA MAY BE HARMFUL
Until recently, there has been only one official national supplier of medical marijuana in Canada, and as of the writing of this book, the federal government of the United States and the government of the Netherlands also have just one national supplier.The herbal marijuana supplied has been subjected to gamma radiation before being released to patients. The objective is to ensure micro-biological safety by sterilizing any potentially toxic microorganisms that are present, which are of concern because many patients are immune-compromised and especially susceptible to infection. However, an unintended result is the reduction of terpenes, especially monoterpenes, which are especially sensitive to irradiation (Fan and Gates 2001). Just as irradiated food sometimes has a flat or cooked aroma that consumers and objectionable, so patients who are familiar with marijuana sometimes and the dearomatized nature of irradiated medical marijuana so objectionable that they turn to the black market. Moreover, as pointed out previously, there is reason to believe that the terpenes normally in marijuana contribute to its medical efficacy, so that irradiated marijuana may in fact be less healthy.
You can read more about zapping Cannabis here:
Evaluating the Effects of Gamma-Irradiation for Decontamination of Medicinal Cannabis