Dosages are Methylphenidate: 5 to 15 mg orally 2 or 3 times a day Methamphetamine: 5 to 20 mg orally 2 times a day Dextroamphetamine 5 mg orally 2 times a day to 20 mg orally 3 times a day Methylphenidate and amphetamine derivatives are available in long-acting preparations and therefore can be dosed once a day in many patients.