All plans must cover at least two drugs from each therapeutic class of drugs. A therapeutic class contains drugs that are similar based on the disease they treat or on the way they affect the body. Biological products, including insulin and insulin supplies, and smoking cessation drugs are covered under Part D. In addition, plans must cover a number of drugs in certain classes: antidepressants; antipsychotics; anticonvulsants; antiretrovirals; anticancer drugs; and immunosuppressants.