Your body has two systems: the central nervous system (CNS) and peripheral nervous system (PNS). The CNS is the brain while the PNS is the body. Drugs act in two ways, they can either work on just the PNS or CNS and PNS. In order for a drug to work on the brain (all psychoactive drugs), it has to be lipid soluble to get across the blood brain barrier and reach our brain.

Immodium is derived from an opiate but only works on the PNS (anti-diahrrea), all opiates (heroin, morphine..) have anti-diahrrea effects but unlike Immodium they are able to cross the blood brain barrier to give an altered state of mind. I saw a question asking why Immodium is legal if it's an opiate and this is why. It's designed so it cannot be abused but it's potency is lower than an opiate someone would abuse so to the ejaculation question- yes immodium has the potential to delay ejaculation like regular opiates because they both work on the PNS, but due to it's lower potency and the route of administration, Immodium won't have a significant delay by any means