Is it permissible to lie to the doctor if he asks about drinking alcohol or practicing sex, for the sake of concealing oneself, and does that affect the accuracy of the diagnosis?

The basic principle is the obligation to conceal one's sins and not to publicize them, based on the Prophet, peace and blessings be upon him, who said: "All of my Ummah are safe except for those who publicize their sins." It is permissible to disclose a sin for an overriding legitimate Islamic interest, such as someone seeking a fatwa (religious ruling), or confessing to a judge for the purpose of having a hadd (punishment) applied, or informing a specialist doctor to help with treatment if there is a medical and legitimate Islamic interest. However, if there is no overriding interest, then the duty is to conceal one's sin.