Is it permissible for a wife to put a sexual stimulant drug in her husband's food or drink without his knowledge, given his sexual weakness and refusal to go to a doctor?

It is not permissible to treat an adult of sound mind except with his permission. Medical treatment is not obligatory unless its ترك (abandonment) poses a definite danger to the patient's life. In the mentioned case, there is no danger to the husband's life, so it is not permissible to treat him without his knowledge. If the wife cannot tolerate that, she has the right to seek separation from her husband through divorce (طلاق) or annulment (خلع).