A life insurance policy is a trust based agreement in which the policy requested mentions his details and offers to buy a policy the insurance company on basis of his details approves policy. If a person hides an information deliberately that can affect his approval to the policy or increase in premium or call for some investigation, the trust is broken. The insurance company now has all the rights to reject a claim, nullify the policy and forfeit all the premiums submitted and/or file a legal case.
Medical conditions, hazardous nature of service, alcohol or other drugs, family history, etc. all matter a lot and are required to calculate the terms and the correct premium of the policy.
It is always a good practice to reveal everything about yourself while filling a policy request form. All the fields mentioned in the policy forma are important and must be filled in with utmost care and honesty.
If you have got a policy by hiding your medical or any other condition that may have resulted in higher premium or a rejection to policy, inform the insurance company immediately.