Preventive drugs are primarily used to prevent the occurrence or recurrence of diseases. They work by activating the immune system against infections (e.g., vaccines), supplementing deficient substances to prevent related disorders (e.g., Vitamin D for rickets), or long-term use to reduce chronic disease risks (e.g., low-dose aspirin for cardiovascular events). Their core value lies in "preventing trouble before it happens," lowering the probability of falling ill.