Can low dose of Amphetamine cause low level of concentration?

Hello doctor,

I started tablet Amphetamine 40 mg four months ago then lowered it to 15 mg. In the first month, it kept me awake for 12 hours with lots of energy and focus. Lately, the effect has gone, and increasing the dose also did not see much effect. The effect changed at 15 mg as I got high sedation from the first 30 minutes, then four hours of low concentration with low sedation and low energy level. A heavy crash with sedating effects occurred so I have split the 15 mg into two dosages. The initial 30-minute sedation is less and has the same low level of concentration. The crash comes and the second dosage causes higher sedation and less concentration. I talked to a pharmacist and she said it is a rare side effect. I am not sure if my body is rejecting it, if is it a dosage problem, or if am I not getting quality sleep and it is causing my body stress. I split the 15 mg today as well to lessen the sedation and force myself to class and take an exam. I felt it was better to take the exam than not to take it at all. I studied for three hours and went to school and I had no choice but to sleep from the sedation in the parking lot. I woke up feeling better to drive and went back home. I did not take the second dose. I missed the exam and I am hoping I will be able to get by this summer. It feels like I have taken a lot of sleeping pills. Without it, I have poor concentration. Please help.

Thank you.