Most people can completely safely place dental implants if they have normal physical and mental health and proper oral hygiene. Like any operation, implantation requires some preparation with due responsibility from the patient.
An unambiguous prohibition for implantation can be:
Diabetes mellitus without consulting an endocrinologist
Leukemia or connective tissue disorders (e.g. lupus erythematosus)
Age younger than 18-20 years (the jaw is not fully formed yet)
Pregnancy
Taking anticoagulants
Anomalies in the location of the sinuses
Hormone replacement therapy (HRT)