At two different points on the Avenue, you'll be bombarded with large red signs with the RX symbol on them. The men inside in the white coats are pharmacists -- and you won't find anything cheaper in the US.
The thing about Tijuana is that any drug is legal, with or without a perscription -- for personal use only. I know, even as I mention this, that there is a certain percentage of the population that's already got their shoes on and headed for the door to drive down here for their own personal drug du jour, but that's not the point here.
Drugs in Tijuana are extremely cheap -- a fraction of the cost that they are in the US. A friend of mine goes here every month to get some kind of foot medication that costs her more than $500 for name brand in the states, and only costs $50 in TJ. If you are without health insurance and are feeling lucky (drug interactions are your own problem if you misuse this store), you can pick up your needed prescriptions in Mexico and transport them back across for personal use only. Be warned that they -may- check them at the border, and if it is a medication not approved for use in the US, you may not be able to bring it across.
The big red pharmacies are the cheapest places in town, to boot. They speak English well, and will answer your questions about interactions -- but you have to ask. This isn't a hold-your-hand Rite Aid; this is off-the-street.