2 days or more: Phentermine has a half life of 20 hours, so it is basically not having much effect after a day or two. Its completely gone in 5 days. Phentermine is a stimulant usually used in weight loss. Since Ritalin (methylphenidate) is a stimulant too, you should not be on both at the same time, nor is it good to alternate them.
Answered 9/28/2016
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