Purity: Using Water Is Harmful
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 658:
If using water would result in a person dying, or it would make him ill, inflict him with some defect, prolong an illness he has, or make his illness worse or difficult to treat, then in all these cases, he must perform tayammum. However, if one can reduce the harm of using water – for example, by heating it – he must do so and thereby perform wuḍūʾ or ghusl as required.
Ruling 659:
It is not necessary for one to be certain that water is harmful for him; rather, if he deems it probable that it is harmful, in the event that his deeming it probable would be considered by people to be reasonable, he must perform tayammum.
Ruling 660:
If someone is certain or deems it probable that water is harmful for him and performs tayammum, and before prayers he realises that water is not harmful for him, his tayammum is void. If he realises this after prayers, he must perform the prayer again with wudu or ghusl unless performing wuḍūʾ or ghusl while being certain of harm, or deeming it probable, would cause anxiety which would be difficult to endure.
Ruling 661:
If someone who is certain that water is not harmful for him performs ghusl or wuḍūʾ and afterwards realises that water was harmful for him, his wudu or ghusl is void.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)