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Andreas Zimmer at the University of Bonn, Germany, and his colleagues studied mice aged 2 months (young), 12 months (mature) and 18 months (old). They implanted mini-pumps under the mice’s skin, and for 28 days these released either low doses of marijuana’s main active compound, delta-9 tetrahydrocannabinol (THC), or a solution without the drug. After this period, the THC-treated mature and old mice performed as well as untreated young animals in memory and learning tests, whereas THC-treated young mice performed considerably worse.