How can a medication be metabolized by a liver enzyme, yet also inhibit it at the same time??

Question

Wouldn't that somehow counteract the effectiveness of the drug, being that you need these enzymes to process the medication??? What happens, is that a reason for side effects?

Answer 1

There is a host of enzymes (called "cytochrome P450 isozymes") in the human liver that is involved in drug metabolism. The isozymes come with funny names like CYP-3A4, 2D6, etc, and they have unique actions on drugs. For example, phenytoin (an anti-seizure medication) is mainly metabolised by CYP-2C9, but it also induces CYP-1A2. This could lead to major drug interactions (not side effects), if the person is also taking another drug that is metabolised by CYP-1A2.

There is a drug called carbamazepine (used for seizures, pain, or psych problems) which induces its own metabolism and is referred to as the "autoinducer". Yes, it does counteract its effect especially when you first start taking the drug. However, autoinduction is usually completed after 3 to 5 weeks of a fixed dosing regimen and should not be a concern afterwards.

Answer 2

Most biological reactions are reversible reactions. They stop after an equillibrium is achieved