Don't bring any drugs unless you have a prescription for them. You will seldom hear of stomach problems from tourists in Baja. The difference is in the water. Water in mainland Mexico comes from rivers, whereas water in Baja comes from wells. Because of the change from your home area, don't use tap water for drinking or brushing teeth.
Bottled water is easily available.
Food bought in restaurants or from some street vendors is quite safe. Fresh fruits and vegetables when washed are also quite safe. No immunizations or vaccinations are presently required for Mexico. In general, biting bugs are scarce. If you have a strong allergy to insect stings, you should bring your own personal anti venom kit.