I was on Tetralysal for about 9 months too. I found that I built up as resistance to it and had to move on to a different antibiotic.
I also had an annoying side affect that made the tablet repeat on me throughout the morning (I honestly wouldn't be able to tell you what the taste was) but I was sure it was the tablet but I just soldiered through that in the hope for clear skin.
When the Tetralysal was working for me I did have clear skin (I did combine it with Duac cream also) but unfortunately like Paul I had to stop and change tact. I drank milk quite a lot also as I'm a fiend for drinking water and I never found it to impair the efficiacy of the drug.
Since then I've been on ...
Tetralysal & Duac Cream (Built up a resistance)
Zineryt Topical Lotion (Not strong enough to get rid of my acne)
Doxycycline (Built up a resistance again)
Trimethoprim (Allergic reaction to this one so wouldn't be able to tell you if that one worked or not)
& now my derm is putting me on Accutane in May.
So in all honesty if this is your first go at attacking your acne then you've got alot of other options should it fail.
Just be positive and stick with the courses - things won't transform overnight and maybe keep a picture log so you can see the changes as that always helps me to see the difference.
Just my 2 cents