It can be difficult to predict medication supplies. The pharmaceutical industry must meet strict quality standards, and drug shortages can happen (e.g., if a batch is rejected for some reason).
Fortunately, the same medication is often made by many companies. If your medication is completely out of stock, your pharmacist can substitute it with an equivalent one. When making a substitution, your pharmacist will ensure that the new drug is in the same family as your prescribed medication. He or she will also tell you how to take it properly and notify your doctor of the new prescription.