This is one of the most common questions that a pharmacist receives and the answer can be lengthy.
The point I always try to get across to people is the difference between a brand and generic drug is NOT like the difference between a box of Fruit Loops and a store brand. The FDA requires rigorous 'bioequivalency' testing to ensure a generic drug is within a certain statistically acceptable margin for both the rate and extent of absorption of the drug.
I would also like to point out that one of the favorite responses a pharmacist will give you when you get a generic drug for the first time is: "This is the same medication you have been taking but it may look different because it is coming from a different manufacturer." How can the medication be the same if it looks different? The answer is they aren't the same, but they are close!
Generic Vs. Brand
A drug is considered generically equivalent to the brand name if the generic is bioequivalent to the brand name drug.
Bioequivalent means that the rate and extent of absorption (i.e. how fast and how much) is the same for the generic as it is for the brand name.
So per the definition by the FDA, the generic drug has to absorb just as fast, and as completely as the brand name drug. Now nothing is ever perfect (even different batches of brand name drugs differ in their exact percentage of active ingredient).
The FDA has acceptable ranges for bioavailability between brand and generic drugs, and that explanation goes beyond the scope of this answer (as it involved some pretty in-depth statistics such as confidence intervals etc...).
Suffice to say, the brand and generic versions of the same drug are pretty darn close. In fact, generic medications have the same quality standards in terms of identity, strength, purity, and quality as brand name manufacturers.
Brand and generics do look different though. This because the inactive ingredients can be different (inactive ingredients meaning the ingredients that have no effect such as fillers and binders in the tablet). These different inactive ingredients are taken into account during testing.
So when do you stick with brand only drugs? One example of a situation where it may be better to stay on the brand is if you have been taking the brand for some time with good effect or the drug you are taking has a very narrow range between effectiveness and toxicity (such as some anticoagulants).
SummaryThere is no known drug interaction between Tamiflu and antibiotic medications. They can safely be used together.