Notwithstanding the provisions of any general or special law, regulation or requirement to the contrary, a health insurance policy or health care service plan contract, that covers prescription drugs shall not limit, reduce, or deny coverage for any drug, if prior to the limitation, reduction, or denial of coverage:

(1) the insured was using the drug;

(2) the insured was covered under the policy or contract; and

(3) the drug was covered under the policy or contract.

A limitation, reduction, or denial of coverage includes removing a drug from the formulary or other drug list, imposing new prior authorization or other utilization management tools, or placing the drug on a formulary tier that increases the patient's cost sharing obligations or otherwise increases the patient's cost sharing obligations.  

Nothing in this provision shall prohibit an insurer from making uniform changes in its benefit design that applies to all covered drugs or from increasing cost-sharing obligations merely due to a percentage coinsurance payment that necessarily increases with an increase in the underlying drug prices.