I have transited through Singapore 40ish times in the past 7 years and have never, not ever, been even remotely hassled by customs. In fact 9 times out of 10 there is no one there. Occassionally you seem them, but if you do they are random checks, and the targets are usually Indians I've noticed (for whatever reason).

A good friend of mine worked at immigration at Changi a few years back. He told me a few points about how things work:

Do not take the lack of visible customs at Changi as meaning you are not being closely watched... There are cameras EVERYWHERE and a wall of people watching them. Plus, luggage is periodically screened (i.e. x-rayed) behind the scenes before it gets to the belt.

If they deem you to be acting suspicious you will be grabbed at or before the luggage carrousel and taken to a back room. You probably wouldn't even notice it happening to someone. They come up to people quitely and say "excuse me sir, would you mind coming with us for a moment, sorry for the inconvenience ya", in typical Singapore politeness...

If your luggage is deemed suspicious, agents (most likely plain clothes but not necessarily) are waiting around the baggage area, watching for who takes the bag, and then off you go to a back room for interrgogation - Singapore style - always polite but death by hanging if you are found with even a sliver of heroin! I'm sure the Singaporeans even apologise as they release the trapdoor lol.

As for prescription drugs - I wouldn't worry about it. As for anything harder you'd have to be mentally retarded to even think about it.