'Placebo Effect', written by Gary Russell, features the Eighth Doctor and Sam.
Back Cover Blurb
It is 3999. An artificial planetoid, Micawber's World, is hosting the Intergalactic Olympic GAmes, and athletes from all the worlds in the Galactic Federation are coming to take part. But when the Doctor and Sam arrive, murders soon begin...
The Doctor finds himself drafted in to examine some bizarre new drugs that are said to enhance the natural potential of the competing athletes. But what is their real purpose? Why are members of the Security Forces disappearing randomly? And just why is Chase Carrington, manufacturer of the drug, so protective of his company's secrets?
Watching and waiting, at the very heart of Micawber's World, is a race of parsites the Doctor has fought before. The Wirrrn have come to the Milky Way from Andromeda, determined to spread their seed throughout a whole new galaxy, and it seems to Sam that the Doctor has his hands too full to pay their threat full attention.