I wouldn't recommend medication - most of the things that they would give you to focus are addictive also.
I wrote a post about the my
psychological reasons for quitting today. Memory loss is one of the things I experience heavily.
You might try a different way of absorbing the material, like taking notes while reading, making a cd and listening or something else.
The short term memory loss I associate with smoking weed is a lot different. I will forget what I did the previous evening, not remember the plot of a movie, or not recall meeting a person I just met 10 minutes ago. Almost like a long term memory black out. This effects me mostly when high, but it improves quickly after a few days off.
You can work on your memory skills through practice. Maybe BrainAge on the nintendo Ds?