These are less frequent than pharmacokinetic interactions and may involve competition for receptor sites which can be antagonistic or additive/synergistic. An obvious example of an antagonistic interactions is a beta blocker such as propranolol taken with salbutamol, a beta 2 agonist. The bronchodilator effects of the salbutamol are blocked by the non-specific (beta 1 and beta 2) beta blocker. Another example in the same area would be anticoagulants with vitamin K. Think of an additive interaction where the effects of one drug are enhanced by another. You may have said that any drug in the same class as another will be potentially additive and that is often the case. One simple example is the potassium sparing diuretic spironolactone with ACE inhibitors which are also potassium sparing where this can lead to hyperkalaemia. Alcohol plus any CNS depressant drug can exhibit additive effects. Looking at our definition of a drug interactions these are not strictly speaking one drug interacting with another but it is important to consider when counselling patients on their medicines.

Susceptible patients

As with ADRs there are patient groups who may be more susceptible to drug interactions. However, unlike with ADRs it is not as easy to classify. The most obvious group are those taking a number of medicines – polypharmacy – which may be unavoidable but is always worth reviewing regularly. The elderly, those with renal or hepatic disease, the seriously or chronically ill patient and those with acute illnesses but serious conditions such as those taking anti rejection medication after a transplant are all at risk. The biggest factor is polypharmacy. Genetic factors may also play a part in susceptibility of certain patients to drug interactions. As with ADRs metabolism is the most likely focus. Those who are unable to metabolise one drug may be more susceptible to the actions of a second drug. See also chapter 6 on specific patient groups and chapter 8 on adverse drug reactions.

Specific Drugs

Look at the BNF Appendix 1 again, you can see that some drugs have more black dots (potentially serious) next to drugs in the list below them than others. Look for example at Lithium then at MAOIs which you looked at earlier. You can see that the majority of drugs taken with lithium or the Monoamine Oxidase Inhibitors have interactions that could be potentially serious. Whereas, something like metoclopramide has only one listed (ciclosporin) as potentially serious. Have a look at drugs you commonly use in practice and see which potentially serious drug interactions are possible.