I wrote a post on this in a different thread, but thought to let my experience be known...

I've flown into the following international airports for varying length of stays between 3-21 days and most of them multiple times:
Reykjavik, Iceland
Glasgow, Scotland
London Heathrow
London Gatwick
Dublin, Ireland
Paris, France

Without exception, I was carrying a minimum of 1400 - 3000 mgs of methadone (yes, I did have a prescription) in my carry-on luggage when I went through Customs. I have arrived early morning and as late as midnight. Always, in these airports, Customs was fully on duty.

The first time I went through international Customs in London with my medication, I declared it. I also carried with me a letter from my doctor which stated my need and prescription. They did phone my doctor's office (at a charge of about $20 to me) to verify it and they also did a quick search of my luggage. They told me I need not declare my methadone in the future.

I never was nervous about it again, and safely passed through Customs without declaring my meds (or my purchases/cigarettes--I always exceeded the limit allowed when I would stay longer than a week--/food products--sorry, I love Alpin cereal and Scottish smoked salmon!) and passed through their agricultural dogs, drug dogs, etc. without incident--even during the Foot & Mouth crisis and Mad Cow disease periods. In fact, Customs is far more scary and stringent coming back into the US than it is going into the countries I have visited.

I'd only be worried if you were trying to carry in an illicit drug or lots of cash!