From which airport are you flying in Europe? Do you plan to bring it back with you onto the aircraft?

It doesn't work the same way at many European airports ie declaring an oversized item isn't a guarantee that it will be allowed. (Items with prescription yes, but not 'other' items necessarily) As N1120A points out, you may need a prescription. Just this week I saw 'declared' contact lens fluid and over the counter cough syrup disallowed at LHR. (But LHR has a Boots in each terminal where you could probably purchase replacement, or just stop at an optician during your travels to see if there is a smaller size/similar product)

Having said that, I would put it IN your liquid bag on the way home. (Yes, all liquids must be in a bag, and the bag pulled out, unlike the varying rules with TSA) At 3,5 oz it is 3,5 ml over the limit, and most likely won't prompt another look.