PTCB Test Prep Therapeutics

Top 20 PTCB Drug Interactions to Know!

Jun 26th, 2020
ptcb drug interactions

Drug Interactions for the PTCB Exam

Questions on drug interactions are common on the PTCB exam. Of course, given the tens of thousands of potential drug interactions, students are not expected to know every single one.

That said, students are expected to know the most common drug interactions for the PTCB exam.

There are different kinds of drug interaction:

  • Drug-drug interaction – where two medicines collide to produce either a desirable or adverse effect. For example – taking aspirin with NSAIDs increases the risk of bleeding.
  • Drug-food interaction – where a medicine interacts with food or drink, again, that produces either a wanted or unwanted effect. For example – taking grapefruit juice with statins increases the risk of adverse effect such as muscle pain and rhabdomyolysis.
  • Drug-disease interaction – where a medicine has a harmful effect on the progress of a disease. For example – beta-blockers can assist patients with heart disease, but they may worsen asthma in the same patient.

In the table below, we review all three kinds of drug interactions for the PTCB exam.

Drug interactions are not random. There is always a mechanistic or clinical reason for the interaction to take place. And as we alluded to earlier, not all drug interactions are clinically negative outcomes.

For example – ritonavir is a drug used in the treatment of HIV/AIDS. However, ritonavir not used to directly treat the condition, but instead it is used to inhibit liver enzymes that metabolize other HIV drugs, such as protease inhibitors. This means that – when protease inhibitors are taken with ritonavir – a lower dose of protease inhibitors can be used to achieve the same clinical effect, whilst also reducing the risk of side effects. Ritonavir is used to “boost” the effects of other protease inhibitors.

A second example is antihistamines. First-generation antihistamines are likely to cause sedation. For many patients, this is a desirable effect as they may have trouble sleeping due to their allergy symptoms. Second-generation antihistamines, such as fexofenadine, do not cross the blood-brain barrier and so do not cause this sedative effect.

These are the kinds of drug interactions that get examined on the PTCB test.

Must-Know Drug Interactions!

Below, we have put together a neat table on the most common drug interactions for the PTCB exam to know.

Of course, this is not a complete list – there are many other drug interactions that students should learn. But this is a convenient starting point if you are struggling to get to grips with drug interactions.

As always, the more you understand the pharmacology of medicines, the greater appreciation you have for why specific drug interactions take place. It demystifies the subject – making interactions appear less random and offering a structure that you can easily remember.

If you would like access to our complete study of drug interactions, take a few moments to become a registered member of PTCB Test Prep. There, you also have access to PTCB practice test questions to help you review your knowledge of must-know drug interactions.

Statins and
Grapefruit juice
Grapefruit juice contains furanocoumarins which interact with enzymes that reduce statin metabolism – increasing the risk of side effects such as muscle pain and aches.
Antiplatelet drugs Clopidogrel
Ibuprofen Anticoagulant drugs
Antiplatelet drugs, NSAIDs, and anticoagulant drugs individually increase the risk of bleeding. Therefore, the risk of bleeding is even greater if any of these drugs are taken in combination with one another. Furthermore, other drugs – such as SSRI antidepressants – also increase the risk of bleeding with these medicines.
Multivalent ions
Levothyroxine Fluoroquinolones Ciprofloxacin Moxifloxacin Bisphosphonates Alendronic acid Pamidronate Tetracyclines Tetracycline Minocycline
Multivalent ions interact with many medicines, reducing the ability of drugs to carry out their therapeutic goals. Multivalent ions include calcium, magnesium, iron – and are also found in milk and antacids. Multivalent ions reduce the effectiveness of drugs such as levothyroxine, the antibacterial drugs tetracyclines and fluoroquinolones, and the osteoporosis drug class, bisphosphonates.
Nitrates and
PDE5 inhibitors
Nitrates cause hypotension and PDE5 inhibitors – such as sildenafil and tadalafil – also cause hypotension. Therefore, there is a significant risk of severe hypotension if both drugs are taken together.
ACE inhibitors
Potassium-elevating drugs
ACE inhibitors increase potassium levels, raising the risk of hyperkalemia. Taking ACE inhibitors with other K-elevating drugs then – such as trimethoprim or potassium supplements – should be avoided.
Protease inhibitors Lopinavir
Ritonavir inhibits CYP enzymes that otherwise metabolize protease inhibitors. This means less dose of protease inhibitors can be used to achieve the same clinical effect without causing excess side effects.
Calcium channel blockers
Beta-blockers – such as metoprolol – should be avoided with calcium channel blockers (verapamil/diltiazem) as the combination increases the risk of heart failure and a slow heart rate (bradycardia).
Broad-spectrum antibiotics
Broad-spectrum antibiotics – such as penicillins and cephalosporins – kill gut flora which otherwise produces vitamin K. This means that these antibiotics increase the risk of bleeding with warfarin.
Loop or thiazide diuretics
Loop diuretics (furosemide) and thiazide diuretics (hydrochlorothiazide) increase the risk of digoxin toxicity by causing low potassium levels.
Loop diuretics
Aminoglycosides (gentamicin, amikacin) cause ototoxicity and kidney damage. Taking aminoglycosides with loop diuretics (furosemide) increases the risk of these adverse effects.
First-generation antihistamines cause sedation. Taking antihistamines with other drugs that cause sedation – such as those listed – increases the risk of sedation.
Some drug classes prolong the QT interval – meaning that they increase the risk of heart rhythm problems, arrhythmias. Taking any combination of drugs in those listed increases the risk of QT prolongation as each of these drugs/drug classes individually carries this risk.
Corticosteroids increase the risk of peptic ulceration and GI bleeding when taken with NSAIDs.
MAO inhibitors
Tyramine is found in foods such as fermented products, aged meats and cheese, alcohol, soy sauce, and many others. Taking foods with tyramine increases the risk of hypertensive crisis when taken with MAO inhibitors.
Omeprazole – a proton-pump inhibitor – reduces the antiplatelet effects of clopidogrel.
Macrolides – such as clarithromycin – inhibit CYP enzymes that metabolize statins. Therefore, the risk of adverse effects of statins increases when taken with macrolides.
Each of these drugs can individually cause hypoglycemia. Taken together, in any combination, this increases the risk of hypoglycemia further (or any other drug class that lowers blood glucose levels).
Phenytoin induces CYP enzymes and so reduces plasma concentrations of drugs such as warfarin, estrogens, and progestogens – reducing their clinical effects.
Beta-2 agonists
Corticosteroids increase the risk of hypokalemia (low blood potassium levels) when taken with beta-2 agonists such as albuterol and terbutaline.
Alpha-blockers include drugs such as terazosin, prazosin, and tamsulosin – all of which can lower blood pressure. Taking alpha-blockers with other drugs that lower blood pressure can produce an additive effect that results in dangerously low blood pressure. This principle applies to any two (or more) drug classes that reduce blood pressure.

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PTCB Test Prep Author


Elaine Walker

Elaine joined PTCB Test Prep in 2017, currently serving as the lead product development manager overseeing both course development and quality improvement. Mrs. Walker is a graduate of California State University and has worked as a pharmacy technician for over twenty years – with particular interests in pediatric pharmacy, extemporaneous compounding, and hospital pharmacy. Over the past 8-years, she has helped prepare thousands of students for the PTCB examination.