Call at : 1-323-455-4676, Strongest pain killer which are commonly known as analgesics. The common pain killers are Morphine which is one of the strongest. Diamorphine is form of morphine that gets dissolve in water and administer via injection. Fentanyl is strong pain killer that is commonly known as Durogesic. The most common form of administration of this drug is through patch on skin. Propoxyphene does not cause the stomach upset but relieves pain.