People who have had experience with such situations have related that the following are among the signs of a person who is possessed by jinn (or Satan):

1.

Strong repulsion when hearing Qur’aan or Aathaan (call for prayers).
2.

Episodes of losing consciousness and/or epileptic attacks, especially when Qur’aan is recited for the possessed person.
3.

Frequent nightmares during sleep.
4.

Tendency to avoid people accompanied by out-of-the-norm behavior.
5.

The jinn who possesses him might speak when Qur’aan is recited for the possessed person.
6.

Madness, as stated in the Qur’aan (interpretation of the meaning): "Those who devour usury will not stand except as stands one whom Satan by his touch hath driven to [epileptic] madness—"2:275

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As for curing this condition the following steps are recommended:

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5. Cupping--removing excess blood.