Treating mental illness by administering synthetic isotope-modified lithium compounds: (i) with the lithium-6 isotope making up at least 95% of the total number of lithium atoms in the compound to treat depression and other mental conditions with reduced alertness levels, (ii) with the lithium-7 isotope making up at least 95% of the total number of lithium atoms in the compound to treat mania and bipolar disorder and other mental illnesses which respond to lithium compounds with natural isotope abundance levels, (iii) with the lithium-6 isotope ranging from 10% to 95% of the total number of lithium atoms in the compound individually tailored to treat patients with a broad variety of mental disorders, including those in (i) and (ii) above.