I personally don't believe this is the case. I have read many times the Kirkland brand is exactly the same as say Pfizer. However, I have used a few different brands and Kirkland is the only one I have had an adverse reaction to.

If the ingredients are exactly the same and at the exact same quantities. Then I can only think of two reasons for this adverse reaction.

1) They are using inferior versions of these ingredients.

2) I'm not exactly sure how Rogaine, Kirkland etc are made but maybe this plays a role not dissimilar to vodka productions yielding different results.

Spex, In the past I've read that Costco stock Rogaine foam, it used to be something like $40 for a 3 months supply.