Is it permissible to give medicine to a patient without his knowledge or to lie to him regarding his treatment?

It is not permissible to lie to an adult, sane patient and give them medicine without their knowledge, unless leaving the medicine poses a certain danger to their life, such as someone who is bleeding. In such a case, treatment is obligatory to preserve life, as Allah Almighty says: "And do not kill yourselves. Indeed, Allah is ever, to you, Merciful" (Quran 4:29), and His saying: "And do not throw [yourselves] with your [own] hands into destruction" (Quran 2:195).