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There is always a possibility that he may do this. There is no way to stop him from attempting this. However, the chances of him succeeding are slim.
You are referring - no doubt - to the doctrine that a person who does not mental capacity when signing something is not bound by it. The law of PA makes clear that a contract is created where there is mutual assent to the terms of a contract by the parties with the capacity to contract. Taylor v. Stanley Co. of America, 305 Pa. 546, 553, 158 A. 157 (1932).
However, the contract is presumed valid. It is up to them to file a complaint for declaratory judgment to void it, and they would have to prove by CLEAR evidence that they lacked mental capacity. This means medical experts, witnesses, etc. And the drug had to have been so powerful that they really did not understand what they were signing.
Most Judges/juries are skeptical of such claims, and barring expensive witnesses and a lawsuit, this would likely fail.
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