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Avoiding treatment failure due to antacids

Professor Hugh McGavock continues his series on therapeutic banana skins with a look at how common indigestion remedies can cause significant interactions

Professor Hugh McGavock continues his series on therapeutic banana skins with a look at how common indigestion remedies can cause significant interactions

Complex modern drugs have a variety of chemical radicals which readily interact with other compounds in the body. Indeed, it is exactly these properties that enable drugs to bind to their intended target to achieve their therapeutic effect.

Unfortunately, a number of important drugs react chemically with all antacids containing calcium, aluminium or magnesium, if any of these are present in the stomach or duodenum when the drug is swallowed. This reaction alters the drug's structure and either prevents its absorption from the intestine or renders it inactive at its target site in the body – heart, liver, kidneys and so on.

The inevitable result is therapeutic failure – and, unless you are aware of its cause, you will not recognise the problem. The box on the right lists the most important drugs that must not be taken if there is antacid in the stomach. A glance at the long list of potential interactions illustrates how significant the issue is.

Many clinicians are unaware of the risk, and since antacids are so widely consumed by patients who ‘don't think it worth mentioning' to the doctor or nurse, the problem needs determined action to prevent it.

41228252In the light of this information, it is clearly essential that prescribers warn patients not to take antacids just before or at the same time as their prescription drugs. Pharmacists, too, need to reinforce this message at every repeat dispensing, and to ask patients about prescription drugs whenever they sell an antacid over the counter.

If the drugs highlighted in the box are taken before a meal, there is no reason why an antacid, if needed, should not be taken afterwards, when it will not affect drug absorption. For those curious enough to wish to understand the antacid-drug interaction involved, there are four:

• formation of a chemical complex – complexation

• adsorption of the drug by the antacid

• resin binding

• destruction of a drug's acid-resistant coating due to the increased pH in the stomach that results from the antacid.

So next time you encounter treatment failure, don't increase the dose without asking the following questions:

• Is it due to non-compliance?

• Is it due to an antacid?

Professor Hugh McGavock is visiting professor of prescribing science at the University of Ulster and course organiser of GP continuing clinical education at the Northern Ireland Medical and Dental Training Agency

This is an extract from Pitfalls in Prescribing and How to Avoid Them. Pulse readers can buy the book at the specially discounted price of £15 + P&P (usual price £18.99 + P&P).

To claim the discount go to Radcliffe Publishings website and enter the discount code PPLSE9 at the checkout. Alternatively, please order via 01235 528820 quoting the same code. Offer ends 28 August 2009.

Antacids drugs antacids

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