Loperamide is an inexpensive μ-opioid receptor (MOR) agonist whose central action is normally restricted by low systemic exposure and ATP-binding cassette subfamily B member 1 (ABCB1/P-glycoprotein) efflux. Its major metabolite, N-desmethyl loperamide (dLop), is an informative matched molecular pair: dLop retains high MOR affinity but inhibits hERG less potently than loperamide, while remaining strongly effluxed and retaining other liabilities. We propose that the scaffold can be redesigned only if four overlapping maps—MOR function, ABCB1 transport, cardiac ion-channel inhibition, and developability—are measured separately and then recombined. Ten modification axes are analysed, with their evidence, favourable and adverse consequences, interaction logic, and synthetic burden. Single-variable pairs establish attribution; higher-order candidates combine only validated, non-redundant modules. We also compare original-plus-original, modified-plus-original, and modified-plus-modified products, while rejecting transporter or metabolic inhibition as a shortcut to brain exposure. Case reports of dependence, withdrawal-motivated misuse, malignant arrhythmia, and death motivate a co-primary abuse-liability programme and layered prevention strategy. The lowest-complexity programme is a dLop-centred, common-intermediate platform that first combines one safety module, one disposition module, and, only when justified, one efficacy-shaping module. The framework fails if improved central exposure or cardiac selectivity cannot be separated from lost MOR function or from respiratory, behavioural, gastrointestinal, addiction-related, and cardiac toxicity.