When two or more OTC or prescription drug interact or react with each other making the medicament either less effective, or dangerous to health, it is called drug- drug interaction or simply drug interaction. You may have heard that doctors often give special advice to people with high blood pressure that they should avoid cold medications containing decongestant as this may cause increase in the patient’s blood pressure.

How does NClex tests your knowledge on drug interaction ?

They may give you some hypothetical question about a patient with High Blood Pressure and patient approaching you for the advice regarding intake of medication during cold.

One of the options could be to avoid using decongestants as far as possible.

Having said that, remember, drug interaction is not always bad.

Some drug combination can complement or increase the effect of other. Example : penicillin  and probenecid combination drug therapy.

We will go through some important examples in drug interactions in this chapter-

Example 1:

Probenecid blocks excretion of penicillin so, penicillin stays in blood or a longer period of time. This is actually helpful as it can be helpful to decrease the dose or improve the efficacy of the dose of penicillin given.

NClex doesn’t test your deep knowledge on any specific drug but most important interactions and drugs in general.

Example 2:

Do you know why Aspirin and codeine are given in combination ?

They can actually give a greater relief in pain management.

Taking about interaction of aspirin with codeine, Let’s talk a little bit about the adverse effects too. It’s safe to take aspirin as a painkiller with paracetamol or codeine. But we should be careful with administration of aspirin with ibuprofen or naproxen without talking to a doctor.

Example 3:

Aspirin, ibuprofen and naproxen belong to the same group of medicines called non-steroidal anti-inflammatory drugs (NSAIDs). The dose of aspirin may have to be reduced in such case.


Which of the following is vital to be informed to the patient if he is taking phenytoin ?

Your options are:

  1. Do not walk in the sun after taking the medicine
  2. Do not take milk at the same time.
  3. Do not swim until the dose is complete and doctor advices you.

If you are unsure, phenytoin is used to control seizures (convulsions), including tonic-clonic (grand mal) and psychomotor (temporal lobe) seizures, in the treatment of epilepsy. As a registered nurse, you may be familiar with phenytoin administration basically during the brain surgery as it can reduce the seizure.

The correct answer to this question is –’do not take milk at the same time’. Nutritional supplements and milk interfere with the absorption of phenytoin.

Example 4:

I want to give you some another advices for phenytoin absorption.

  • Phenytoin is compatible only with saline solutions; dextrose forms an insoluble precipitate with phenytoin.
  • Carbamazepine decreases blood levels of phenytoin and hormonal contraceptives. Do not concomitantly administer phenytoin with carbamazepine.
  • Phenytoin interacts with hormonal contraceptives one of the main reasons behind failure of the hormonal contraceptives with carbamazepine is due to the drug interaction.


As soon as you see the drug phenytoin or Antacids or term ‘milk’ in your examination, remember there could be question or chances of interaction.

Let us see some other cases where milk should not be administered to the patient concomitantly:

Example 5:

Tetracyclines can interact with calcium and magnesium thus causing an inadequate absorption of tetracyclines. Milk contains calcium so we should be cautious in administration of tetracyclines with milk.

Examples of tetracyclines are: lymecycline, methacycline, minocycline, rolitetracycline, and doxycycline.

For full articles and over 3 hours of pharmacology crash course, click the link here. (Get in-depth overview and more than 100 most important drug interaction examples through this link).

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