Medicine can easily obtain for a relatively low cost, however the effectiveness of the drug and the cleanliness and expiration date can be misleading, come with your own prescriptons. As careful as you are, it is nearly impossible to avoid gastrointestinal illnesses along the way. Bring a guide book, I recommend the lonely planet, it is an excellent resource and you will uses it often, there are copies in the library, but your own copy will be invaluable. Cultural taboos include not showering after dinner, not eating cold fruit after dinner and not crossing your legs at a baby shower. Eye contact also means very different things in different situations.
Men are very aggressive and eye contact can be an invitation for unintended contact.
Never take a non radio taxi at night and never walk alone at night. Always wear backpacks in the front and never carry money in your back pocket. Put money in different locations, a money belt is a very good idea, especially if you are traveling with your passport, which you often will be doing. Avoid the police and the men with the large guns, they will often not help you, but rather harm you Mexico is an incredible journey I learned so much there that I will constantly be amazed that I was able to absorb so much in four months. The friendshipships I are incredible and will last a lifetime.