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If you decide that you want to remove a family member or friend's access to your health data, or that you no longer want access to their data, begin by touching the "Share" tab, then touch and swipe left on the person's name, and finally touch the resulting "Revoke" button. In the screenshot shown below, after touching "Revoke," Fran Demoski would no longer be able to see your data.
They will no longer display under the sharing option their access was revoked from ("SHARES THEIR DATA WITH ME" or "CAN SEE MY DATA").
If a family member or friend revokes access to their data or access to see your data, they will no longer be listed under the appropriate sharing option on the "Share" tab. If they revoke access to their data, you will no longer be able to view their data from the "Share" or "Timeline" tabs.