只要明白这句话的原本组成就能理解了：
“The subjects were given quadruple the normal dosage of the drug.”
是从简单而变成复杂的过程：
1.The subjects were given the drug.（被测试的主体被喂食、注射了药剂）
2.The subjects were given the normal dosage of the drug.
（被测试的主体被喂食、注射了正常分量的药剂）
3.The subjects were given quadruple the normal dosage of the drug.
（被测试的主体被喂食、注射了相当于四分之一正常分量的药剂）
所以,quadruple 是用作形容药剂的分量的,不是用来支持动词 were given 的.
希望说的明白,