I'll put my pharma industry hat on for a minute...

Generics have to have the same active ingredient, but all the other stuff that makes a pill a pill are up for grabs. They have to be non-toxic, but allergic reactions do exist.

My observation is, the more potent the drug is (i.e. the lower the dose), OR the narrower the "window" of efficacy, the more likely that other other ingredients will affect what is called "bioavailability", or how the drug is actually used by the body. Synthroid is a classic example: it is very potent (25-200 micrograms; basically a speck) for dose. Also, just a little too much or too little can cause problems.

Also, did you know that the dose can vary by 10%? So, say you're on 100 micrograms of Synthroid. The range can be 90-110 mics. I think you can see the problem here.

/pharma hat off