I am very concerned about my teenage daughter, who seems listless and out of sorts. Also, she has half a dozen little pin-prick marks and bruises over the veins in her arms. What does this mean?

The pin-pricks suggest she or somebody else has been injecting drugs into her veins. The only alternative explanation is that she's recently had a load of blood tests but that doesn't seem very likely. It's vital you take her to your family doctor for a full examination and a discussion about what has been going on in her life recently.