THIS IS A CONTEST: THE WINNER WILL BE SENT A FREE COPY OF MY BOOK "Challenging Nature."
 
The five chemicals shown in the picture below are all consumed by millions of Americans as a means for increasing alertness (sometimes some are used for other purposes as well).   But each has different effects on the human body in terms of addictiveness, carcinogenicity, acute toxicity, and actual effectiveness, as illustrated.  Based on this information, can you decide which is legal and which is not?
 
Mental Performance Enhancing Chemicals

CLICK IMAGE TO SEE LARGER VERSION
 
Consumption of each chemical has a greater or lesser probability of causing addiction [***** means highly addictive, *** is somewhat addictive, (.) means not addictive if consumed as suggested but could be addictive if consumed by a non-standard means, the blank (5) is not addictive under any circumstances].  Consumption of one chemical (2) is associated with carcinogenicity. This chemical is also associated with acute toxicity, while three others [labeled (*)] are only toxic if consumed in very large amounts.  Finally, four of the chemicals are associated with a greater or lesser effectiveness at providing people with an increase in mental alertness (to perform better on a college exam, for example), while one doesn't do anything in this regard.

OK.  Here's the premise for the contest.  One of these chemicals is legally available only through a difficult-to-obtain prescription (although many college students use it illegally). Another one can only be purchased by adults.  The other three can be purchased legally without age restrictions.  
Which chemicals are in each of these legality classes?



The first person to post a completely correct answer wins the contest.  (If you have a degree in chemistry or biochemistry, you are not eligible.)  Once the right answer is posted, I will reveal what each chemical is.