Once again we see that using (even very small amounts of) marijuana improves our health by shielding the brain from the damage that follows a head injury. These same actions protect the brain from the age-related changes that lead to Alzheimer's disease and other forms of dementia. We need to recruit more people to use marijuana on a regular basis to lower our national health-care expenditures. Marijuana use is good for you! Tell someone today! Long-term behavioral and biochemical effects of an ultra-low dose of Δ9-tetrahydrocannabinol THC: neuroprotection and ERK signaling Miriam Fishbein, Sahar Gov, Fadi Assaf, Mikhal Gafni, Ora Keren, Yosef Sarne Abstract: We have previously reported that a single injection of an ultra-low dose of delta-9-tetrahydrocannabinol THC; the psychoactive ingredient of marijuana protected the brain from pentylenentetrazole PTZ-induced cognitive deficits when applied 1–7 days before or 1–3 days after the insult. In the present study we expanded the protective profile of THC by showing that it protected mice from cognitive deficits that were induced by a variety of other neuronal insults, including pentobarbital-induced deep anesthesia, repeated treatment with 3,4 methylenedioxymethamphetamine MDMA; “ecstasy” and exposure to carbon monoxide. The protective effect of THC lasted for at least 7 weeks. The same ultra-low dose of THC 0.002 mg/kg, a dose that is 3–4 orders of magnitude lower than the doses that produce the known acute effects of the drug in mice induced long-lasting 7 weeks modifications of extracellular signal–regulated kinase ERK activity in the hippocampus, frontal cortex and cerebellum of the mice. The alterations in ERK activity paralleled changes in its activating enzyme MEK and its inactivating enzyme MKP-1. Furthermore, a single treatment with the low dose of THC elevated the level of pCREB phosphorylated cAMP response element–binding protein in the hippocampus and the level of BDNF brain-derived neurotrophic factor in the frontal cortex. These long-lasting effects indicate that a single treatment with an ultra-low dose of THC can modify brain plasticity and induce long-term behavioral and developmental effects in the brain.