Opps.  I said category three, when I should have said category four.  (No comments about showing my old age please.)

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I posted this elsewhere.  Since it concerns questions that are asked so frequently I feel it merits repeating.

Carrying Medication in Tijuana
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If you have a medical condition, carry whatever medication you need in the container the pharmacy provided. (It never hurts to have a copy of your Rx.) Don't carry more than is needed for a few days.

Here is the advice from the US Consolate http://travel.state.gov/travel/cis_pa_tw/cis/cis_970.html:

The U.S. Embassy cautions that possession of any amount of prescription medicine brought from the United States, including medications to treat HIV and psychotropic drugs such as Valium, can result in arrest if Mexican authorities suspect abuse or if the quantity of the prescription medicine exceeds the amount required for several days' use. Individuals should consider carrying a copy of the prescription and a Mexican doctor's letter explaining that the quantity of medication is appropriate for their personal medical use.

Since terms like "abuse" and "a few days worth" are subject to interpretation those spending a lot of time In Mexico might consider spending the $20.00 to $40.00 that seems to be the going fee for a prescription from a Mexican physician.

At a minimum, I suggest carrying all medication in the bottle in which it was sold. This includes over the counter medications. (Think about: how is a cop supposed to distinguish illegal drugs from legal drugs.) A friend told me about being “fined” by the cops when they found him carrying his ulcer medication without a prescription. The same person reported the cops also gave him a hard time because he carried a variety of over the counter medications together with his ulcer medication mixed together in a pill case. While carrying a variety of medications in a little tin was helpful when he encountered medical problems, it placed him in a bad position when he was stopped by the cops who accused him of using drugs illegally.

Buying Medication in Mexico
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There is a difference between what is legal is what is common practice and statutory law. In general you can buy most medications (things likes diabetic and high blood pressure meds) without an Rx and without problems from the cops. The letter of the law, however, required you to have an Rx. It is almost universally ignored. In fact, most pharmacies will tell you an Rx isn't needed.

I personally have been buying medications in Mexico for years without an Rx. I've never had problems with the local police. (But that doesn't mean that someday I won't run into difficulties with an "enterprising cop." )

Bringing Medications into the US
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I’ve been asked by friends in the US if you can buy medications in Tijuana cheaper than in the US and take them back to the US. The first part of the question is easy to answer. Most medications are cheaper in Mexico than in the US. Importing drugs for personal use is a bit more complex. I think I know the laws on this subject; however, I’m not an attorney so it would be best to get legal advice from an attorney or the customs service. I will, however, explain what I would do and comment on my own experiences. I believe it is safest to have a prescription from a US doctor when importing medication into the US. The next best thing is to have a prescription from a Mexican doctor. If I were using medication that had a high “black market” appeal, Valium for instance, I would definitely want an Rx from a US physician. I wouldn’t try importing more than a 60 to 90 day supply of the medication. I have a friend who is diabetic and there are many times I’ve taken him to TJ to get his meds. His Rx has expired. There have been a couple of times he’s received a lecture from a US Customs agent telling him that he should have an Rx; however, he has always been allowed to bring his medications back into the US. (He’s a middle aged WASP who dresses and looks like a businessman on his day off.) He and I always declare our medication; we’d rather have it confiscated than risk being arrested for something like smuggling.

Legal Source
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I have heard many people debate the question whether an Rx is need to buy medication in Mexico. So I thought I'd provide my source of information for my comments on this subject.

http://www.salud.gob.mx/unidades/cdi/nom/072ssa13.html
NORMA Oficial Mexicana NOM-072-SSA1-1993, Etiquetado de medicamentos.
5.11.1 En la etiqueta de los medicamentos, se deberá expresar la clave de registro sanitario, tal como se indica en el oficio de registro, seguida del número romano de la fracción que corresponda a los medicamentos, ambos con la misma tipografía y tamaño, y de acuerdo a la siguiente clasificación:
I. Medicamentos que sólo pueden adquirirse con receta o permiso especial, expedido por la Secretaría de Salud;
II. Medicamentos que requieren para su adquisición receta médica que deberá retenerse en la farmacia que la surta y ser registrada en los libros de control que al efecto se lleven;
III. Medicamentos que sólo puedan adquirirse con receta médica que se podrá surtir hasta tres veces;
IV. Medicamentos que para adquirirse requieren receta médica, pero que pueden resurtirse tantas veces como lo indique el médico que lo prescriba;
V. Medicamentos sin receta, autorizados para su venta exclusivamente en farmacias, y
VI. Medicamentos que para adquirirse no requieren receta médica y que puedan expenderse en otros establecimientos que no sean farmacias.

Medications fall into one of several categories (or schedules):

Category I medications may only be taken with permission of the Secretary of Health. Usually they are experimental drugs.

Category II drugs require the pharmacy to keep records on who bought the medication. They will probably keep your Rx and will need a new Rx each time you visit the pharmacy.

Category III medications are limited to three refills. Obviously records should be kept by the pharmacy.

Category IV--> medications require an Rx from a Mexican physician. However, the pharmacy is not required to keep records about who bought the medication.

In many Latin American countries so many people lack money to visit physicians that pharmacists often provide medical care. Hence laws
requiring a person to have an Rx are seldom enforced. The pharmacy may even tell you that you don't need an Rx for Category IV--> medication. However, the law is on the books and an "enterprising cop" can use it to extort money. (Fortunately, this seldom happens.)

Most medications--including Viagra, Cialis, and Levitra--are Category IV medication. (Cialis's registration number, for example, is 181M2002 SSA IV.) The regulation above says these medications require a prescription from a Mexican physician and that it can be refilled as many times as the doctor indicates. Since no recordkeeping by the pharmacy is required, so you see why people think an Rx isn’t required.

Burt's advice is right on the money.

Thanks for reminding me of these things Burt!

FYI, a guy I know was arrested in TJ several years ago for buying a prescription med without a prescription from a Mex. physician.  He had one from a US physician and he was gonna be fine bringing it across.  But he got arrested down there cuz the pharmacy he bought it from was refusing to pay the police not to mess with them.  So, to get even with them, they busted one of their customers.  

He spent months in jail and lost his job.  The case was eventually dismissed but it cost him a fortune.

This was several years ago and one of the posts below suggested this may not be happening now, so keep that in mind.

But I know for a fact it happened on this one occassion.

Having said that, I have bought meds down there without a prescription too many times to count and never had a problem.  I read Burt's advice and my recollection was actually a little different.  My recollection was that basic meds. like insulin, or cholsterol meds. or e.d. pills did not require a prescription.  I remember once having done what Burt did and looked it up and coming to the conclusion that it was only more serious meds, things that could be used as psychotropics and such, that required a prescription.  According to Burt's source, I seem to have been mistaken.