SB 959 amends the Insurance Code to modify rules on prescription drug coverage under health benefit plans. It prohibits insurers from altering drug coverage or cost-sharing for prescription drugs already approved or covered in the prior plan year if the patient remains enrolled in the plan, the drug is prescribed for an ongoing medical condition, and the prescribing provider determines it is the most appropriate treatment. Insurers cannot remove covered drugs, require prior authorization, impose step-therapy restrictions, increase out-of-pocket costs, or move a drug to a higher cost-sharing tier upon plan renewal. However, the bill allows insurers to make changes if the FDA issues safety warnings, the drug is discontinued, or manufacturers remove it from the market. The changes apply to plans issued or renewed on or after January 1, 2026.