Doctor’s prescribe medications based on the most widely successful results with specific ailments, but because each person reacts differently to various substances what works well for a vast majority could worsen symptoms for others. Over the years penicillin has become a commonly used cure for several ailments, but also a caustic allergen to some patients. If a prescription medication makes you feel ill, then you may be allergic to the medication. Explain your symptoms to your doctor. If the doctor says that your reaction to the medication uncommon, then he may adjust the medication or prescribe an alternative.