Anti-radiation drug

The Thals had anti-radiation drugs which they used to survive in the atmosphere of Skaro following the neutronic war. Alydon left a case of them outside of the TARDIS. Not realising what they were, the First Doctor, Susan Foreman, Ian Chesterton and Barbara Wright ignored them.

Susan later returned for the drugs and some were given to the Daleks, who believed that they needed them. However, they found that the Daleks in Section 3, the first to receive the drugs, stopped working and died. They realised that they were now dependent on the radiation to survive. (TV: The Daleks)

During the Last Great Time War, Time Lords attempted to obtain the recipe for the anti-radiation serum from the Thals knowing it would be fatal to Daleks, but due to their pacifist nature they refused to explain it. (PROSE: Dalek Combat Training Manual)