Fast of Gedaliah — in the Process of Withdrawal

Shalom u'vrachah. In my yeshivah there is a young man who was addicted for years to stimulant pills for attention and concentration. In recent years he took very high doses, and over the past month he has been in a withdrawal process, which causes him great fatigue and restlessness. What is his halachic status regarding the fast?

Shalom u'vrachah 

If the fast will cause him weakness at the level of illness — that is, weakness throughout the entire body — he should not fast. 

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