Meta-owned WhatsApp is end-to-end encrypted and messages are stored on your device, so someone else accessing your account on another device cannot read your past conversations. Yet, if you suspect someone else is using your WhatsApp account, you must notify your family and friends as that person can impersonate you in chats and groups.
Never share your WhatsApp SMS verification code with others, not even family or friends. In case, you share your code and lose access to your WhatsApp account, read the instructions below to recover your WhatsApp account. Be cautious of who has physical access to your phone,