With certain types of cover, usually critical illness cover, companies will often allow you the option of making payments that are either guaranteed or reviewable.

Guaranteed - the company you buy cover from agrees never to increase your payments.
Reviewable - you agree that the company you buy cover from can review your payments at set intervals. As a result your payments may change.

Initially, reviewable premiums will usually be cheaper. However, over time they may increase, possibly making them a more expensive option than guaranteed premiums in the long run. However, if you’re on a tight budget now, it may be worth considering reviewable premiums.