You do realise that you're doing something quite dangerous by turning to dopamine-influencing drugs to motivate you, right? This class of drugs can change brain structure, promote cancer growth and generally make you devoid of emotion. At the very least, protect yourself with anti-oxidants and NMDA antagonists. There are plenty of less serious things you can take/do if you simply lack motivation. Exercise, cut out sugar and caffeine entirely and try Noopept.
Also see:
* http://www.nih.gov/news/health/feb2009/nida-02.htm
* http://thelastpsychiatrist.com/2006/05/ritalin_causes_cancer...
* http://www.cchrint.org/2010/11/15/adhd-ritalin-%E2%80%93-bra...
But for the love of god, think about what you are doing to your body. There are other ways, just look a little harder.