There is a big gray area that exists in this matter, plus the lack of information from authorities and government officials, many people don't know if what they are taking with them can put them in some sort of either small or big legal risk.
In a perfect world more efforts would be made towards educating tourists and locals about what items, fruits, vegetables, meats and plants cannot be brought into Mexico.
Is urgent that the Mexican Customs office publicize an easy and comprehensive list that will make a difference, sadly most of the decisions still are made in Mexico City. Historically the main official in charge of the customs office in Tijuana, gets the management position via political favors and not because any knowledge of the laws, the right skills or work experience.
The team of SanDiegoRed did some research to find more information regarding this issue, and put together a list that is not official but helps you to get an idea of what options you have.
According with the Mexican Customs official website, aduanas.sat.gob.mx, this items are restricted: Firearms and ammunition. In order to import firearms and cartridges you must secure an import permit from the Ministry of Economy and from the Ministry of National Defense.
In accordance with the Law of the General Taxes of Import and Export, the following products are prohibited: Alive predator fish, Tatoaba fish, turtle eggs or any class, flour of poppy seeds, drugs, stamps or printed transfers in color or in black and white, displayed for their sale in envelopes or packages, even when they include chewing gum, candies or any other type of articles containing drawings, figures or illustrations that represent childhood in a degrading or ridiculous way, or attitudes of incitement to violence, to self-destruction or in any other form of antisocial behavior.
Also the Ministry of Agriculture, Livestock, Rural Development, Fisheries and Food, better known as SAGARPA, a unit of the federal executive, which has among its objectives the inspection policies and regulations don't have a list posted on their website, sagarpa.gob.mx
Asking around we found that fresh meat, frozen or in a can along with dog food, beans and rice are items that are prohibited to cross to Mexico.
Prepare yourself before you go south; be aware of products that have safety requirements. Don't forget that new ports of entry mean new personnel, which makes border crossing inspections stricter than ever.