“purple heart”の意味は？

BCのコメディ『Only Fools And Horses』を題材にして、ETC英会話のコリン先生のレッスンを受けています。

デルがロドニーにこう言います。
Take one of your Purple Hearts, ‘cause I’ve got a surprise for coming you.
（これから君を驚かせることがある。だから、パープルハートを一つ飲んでくれ。）
さて、このパープルハートとは何を意味しているのでしょうか？

Take one of your Purple Hearts, ‘cause I’ve got a surprise for coming you.

Take one of Purple Hearts.Do you understand Purple Hearts? Purple Hearts are speed, drugs make you go fast. In the 60’s, it was very, very common. Now, I’m reading in the paper that English girl singer Amy House, she died of an overdose. But now they take heroin and Ecstasy, all those kind of things. The colour was purple, the shape’s like a heart. But it was speed, makes people very active. So he says, “Take one of your Purple Hearts.”

■出演
・ETC英会話のコリン先生

■聞き手
・青樹洋文

………

◆使用楽曲 (BGM)
“Toxic Rainfall” of some1else from ccMixter