Worsens Verbal Memory
The long-term consumption of marijuana is linked to poorer verbal memory at middle age. Researchers in Australia found for each additional five years of marijuana exposure, verbal memory was 0.13 standardized units lower than for those who never used it; this corresponds to the average of one of two participants remembering one word fewer from a list of 15 words for every 5 years of use.
The participants’ IQ was assessed at age 13 (pre-marijuana use) and again at age 38. Early initiators and persistent users had the largest decline in IQ scores — 8 points compared to those who had never used or those who had used and then stopped.