Discussions that mention cimetidine

Pain Management board


Hey Phlox, There is usually a warning on the paper work you get about interaction with other meds that use the same liver enzymes to break them down.

West Virginia University School of Pharmacy, Morgantown, West Virginia

Many drug interactions are a result of inhibition or induction of cytochrome P450 enzymes (CYP450). The CYP3A subfamily is involved in many clinically significant drug interactions, including those involving nonsedating antihistamines and cisapride, that may result in cardiac dysrhythmias. CYP3A4 and CYP1A2 enzymes are involved in drug interactions involving theophylline. CYP2D6 is responsible for the metabolism of many psychotherapeutic agents. The protease inhibitors, which are used to treat patients infected with the human immunodeficiency virus, are metabolized by the CYP450 enzymes and consequently interact with a multitude of other medications. By understanding the unique functions and characteristics of these enzymes, physicians may better anticipate and manage drug interactions and may predict or explain an individual's response to a particular therapeutic regimen.

The basic purpose of drug metabolism in the body is to make drugs more water soluble and thus more readily excreted in the urine or bile.1,2 One common way of metabolizing drugs involves the alteration of functional groups on the parent molecule (e.g., oxidation) via the cytochrome P450 enzymes. These enzymes are most predominant in the liver but can also be found in the intestines, lungs and other organs.3-6 These cytochrome P450 enzymes are designated by the letters "CYP" followed by an Arabic numeral, a letter and another Arabic numeral (e.g., CYP2D6).7 Each enzyme is termed an isoform since each derives from a different gene. It should be noted, however, that structural similarity of enzymes cannot be used to predict which isoforms will be responsible for a drug's metabolism.

Drug interactions involving the cytochrome P450 isoforms generally result from one of two processes, enzyme inhibition or enzyme induction. Enzyme inhibition usually involves competition with another drug for the enzyme binding site. This process usually begins with the first dose of the inhibitor,8,9 and onset and offset of inhibition correlate with the half-lives of the drugs involved.9


Until genetic tests for isoform expression become available, a physician can often anticipate drug interactions in a patient by knowing which medications inhibit or induce P450 enzymes.SSRIs and cimetidine inhibit metabolism of tricyclic antidepressants, but the clinical significance of this finding depends on individual genetic variations and concomitant medications.


Enzyme induction occurs when a drug stimulates the synthesis of more enzyme protein,9 enhancing the enzyme's metabolizing capacity. It is somewhat difficult to predict the time course of enzyme induction because several factors, including drug half-lives and enzyme turnover, determine the time course of induction.

If you do a google search for cytochrome p450 you will get tons of info. It's alslso not just drugs that use this enzyme but foods too. Grapefruit juice is a notorious cyp 450 inhibtor.
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I can do a quick search adding Tylenol to the search like Tylenol and the cytochrome p450.
Be right back.

Bye the way, I am feeling better now, It took agood 5 days for things to setle down and I've just been buisy wit my daughter and haloween and yesterday I got to test my standing limits again while waiting to vote. I did manage 90 minutes of alternating from standing, kneeling and sitting on the curb, But was a sweaty mess by the time I got my name checked. I gueess it's good to test your limits from time to time. :eek:
Take care, Dave
PS IF I find anything about Apap and p450 I will let you know