What is the ruling concerning the patient, the doctor, and the medicine, in the case where the doctor takes a portion of a patient's medicine which the latter then sells instead of taking?

The fundamental principle is to interpret people's affairs based on their apparent meaning, unless strong indications suggest otherwise. Therefore, if a patient tells a doctor that they are in pain, it is not permissible to accuse them of lying without evidence, based on the Almighty's saying: "Avoid much [negative] assumption. Indeed, some assumption is sin."

If the patient's lie is proven, or if there is strong evidence for it, then it is not permissible for the doctor to dispense medication to them, because medication is allocated for those in need. It is also not permissible for the doctor to share patients' medications with them, as this is forbidden usurpation, based on the Almighty's saying: "And do not consume one another's property unlawfully."

The doctor must return the equivalent of what they took in medication to the hospital or to the patients if they assisted those who were not in need in taking it, or if they took it from those in need without evidence of their falsehood or lack of need for the medication.