This Hadeeth, which is related by Al-Bukhari on the authority of Abu Hurairah, may Allaah be pleased with him, is authentic, and Abu Dawud recorded it with the addition:
وإنه يتقي بجناحه الذي فيه الداء
"Indeed what is in its wings contains the antidote." (Abu Dawud no. 3844 and Al-Bukhari no. 3320, 5782)
And modern medicine attests to its meaning.
Some people may not feel up to drinking the contents after dipping the fly into the glass, and if they do not feel like drinking, they should not be forced, because a person is not ordered to eat or drink that which disgusts him. In an authentic hadeeth, the Prophet (sallAllaahu `alayhi wa sallam) permitted others to eat a lizard, but he didn't eat it himself: he said,
لم يكن بأرض قومي فأجدني أعافه
"Indeed it is not from the land of my people, so I have a dislike for it." (Al-Bukhari no. 5537 and Muslim no. 1946)