Only if it can be proved that the signature was under duress or that their judgment was impeded enough to consider them incompetent. But generally, I imagine that being a little high is not going to excuse a legal document, unless the document is very outlandish (like signing over a house without payment.)
A signed document in court can be revoked if the person has the medical proof. This is because it shows that the particular person was under the influence of drugs when committing the crime. However, the revocation depends on a number of factors like if the person is in now in good capacity. You need to contact an attorney so as to be advised on the legal procedure that takes place in such a case.