As part of a workshop on clinically significant drug interactions, which of the following inhibits hepatic enzymes?

• A. Omeprazole
• B. Paracetamol
• C. Loratadine
• D. Cimetidine

Answer: (D)

Hepatic enzyme inhibition is a key way drug interactions happen: when a drug lowers the activity of liver enzymes, especially the cytochrome P450 system, it slows the metabolism of other drugs and increases their exposure. Cimetidine is an H2 receptor antagonist known to inhibit several CYP enzymes (such as CYP1A2, CYP2C9, CYP2D6, and 3A4). By dampening these enzymes, it can elevate levels of co‑administered drugs and raise the risk of adverse effects or toxicity.

The other drugs listed are not strong inhibitors of hepatic enzymes. Omeprazole can have some enzyme interaction effects, but its inhibitory impact is weaker and more context‑dependent. Paracetamol is not an enzyme inhibitor; it is mainly processed through conjugation and a separate oxidative pathway, with hepatotoxicity arising from a metabolite at high doses rather than enzyme inhibition. Loratadine is primarily a substrate for metabolic enzymes and is not a meaningful inhibitor of hepatic enzymes itself.

Therefore, the drug most likely to inhibit hepatic enzymes and cause interactions is the one that directly reduces CYP activity.