Yes pharmacist is right. The dose still is in therapeutic range. The drug is said to be expired when there is 10 percent degradation. It means still the concentration of active ingredient is 90 percent. For sale of safety of customers companies show expiry little bit advance. So no need to take tension. It is good habit that you check the expiry of drugs.

In above answer it is sake not sale.for sake of customers....

Thank you Doc Sharma. That is another helpful lesson to me.

Shelf Life - the time period from when the product was manufactured to it’s ‘expiry date’ -i.e. The time period the product is expected to be safe , effective and fit-for-purpose provided it has been packaged and stored in recommended conditions throughout this period,

Expiry Date - all drugs tend to deteriorate /degrade from the point of manufacture and the ‘expiry date’ of a product is the end point of its shelf life taking into account a tolerance of degredation ( normally less than 10%). Degredation is normally due to hydrolysis (chemical degredation by water) or oxidation ( reaction with oxygen) or by microbiological contamination . Drugs produced in liquid forms ( e.g. Oral mixtures , injections) are generally less stable and degrade more quickly than the same drug in a solid formulation ( tablet /capsule) . Degredation is also temperature dependant , and some formulations are designed to be stored at room temp (25 degrees C) whereas some must be refrigerated ( 4–8 degrees C).

Half-life. This is usually a reference to the time taken for the body to eliminate 50% of the dose of drug after the time of administration. It varies with different drugs and between individual patients but ‘average’ half-lives’of drugs may be found in the literature. E.g. Most penicilllins - half lives around 20 mins , digoxin ( a heart medication) - half life = 36 hours - both assume normal kidney function etc.

pharmacist was right and your concern also

Thank you respected colleagues for your responses. So enlightening

Good question! Manufacturers are required to put batches of the drug product up on stability to ensure that the quality attributes are maintained over the registered shelf life of the drug, to ensure safety and efficacy of the medication. In establishing the shelf life, which determines the product expiry date on the label, manufacturers build in a margin to minimize the risk that a quality attribute will fail to meet specification during conduct of stability studies. Product recalls are costly and have a potential negative impact on the manufacturer's reputation as a supplier of quality medicines.

I follow Beemnet Mengesha Kassahun answer.