I agree with Bimini, I don't care for the medication excuse. There are plenty of other ways to say it honestly and simply.
In my own experience, I found that no one except my former drinking buddies particularly gave a damn whether I was drinking or not. Most people are perfectly happy with "yes I'd love a coke/water/whatever" as a reponse to "would you like a drink?" They are being polite and offering a beverage, you can be polite and accept a non-alcoholic one.
You will need to decide if you're still able to be around your former drinking buddies and not drink. Some people can be pretty persistent if they know you used to drink heavily, they will want to keep the "fun" going.
Ultimately you need to decide what YOU want and commit. You cannot let your surroundings and friends choose for you. I let myself believe I couldn't quit for a long time because of my surroundings (I owned a bar--talk about drinking culture!). I was wrong. I can and did quit while still owning that bar (I have since sold it) but I had to change my behavior. I had to be firm in my decision and not let others sway me. Sobriety had to be more important to me than anything else.
I am almost 2.5 years sober and my life doesn't even remotely resemble what it used to, it is infinitely better.
You can do this!!!