Police said they found two joints on 19-year-old Abdullah Morsi and a friend in a car parked by the roadside in Qalyubia province north of Cairo, security officials said yesterday.
They were released pending investigation after agreeing to give urine samples, judicial sources said, adding that the results had not yet been released.
Morsi's other son, Osama, had dismissed the accusations, saying the authorities were "fabricating the case" and that his brother's arrest was an attempt to "defame the family".
Amnesty International says that more than 1,400 people have been killed, mostly Morsi supporters, in street clashes since his ouster.
Morsi himself and several Brotherhood leaders have been put on trial.