Is there any Shar'i or medical evidence for what Imam Ibn al-Qayyim mentioned in the book "Al-Tibb" (Medicine) regarding things that corrupt the intellect and understanding, and what is the extent of the Imam's connection to medicine, especially concerning frequent intercourse and sleeping on one's back?

There is no specific Sharia evidence for the authenticity of the mentioned transmission. Ibn al-Qayyim did not claim that what he established were Sharia truths; rather, his foundation was based on its proof through medicine and its experiences. He had a keen interest in and knowledge of medicine. He indicated that the medicine of the followers of the messengers is more beneficial and healthier, and that the medicine of the followers of their seal, Muhammad (peace be upon him), is the most complete, soundest, and most beneficial medicine. The validity of these matters should be referred to specialists among physicians.