40 Drug Interactions Every Pharmacy Technician Needs to Know. 2026

NSAIDs and Fluoroquinolones

NSAIDs increase the risk of seizures when taken with fluoroquinolones, such as ciprofloxacin.

Grapefruit Juice and Statins

Grapefruit juice increases the risk of muscle cramps / myopathies when taken with some statins.

Omeprazole and Clopidogrel

Omeprazole, a proton-pump inhibitor, reduces the antiplatelet effect of clopidogrel.

CNS drugs and Alcohol

Alcohol enhances the sedative effects of drugs that affect the central nervous system, such as benzodiazepines or SSRI antidepressants.

Antacids and Tetracyclines

Antacids impair the therapeutic effects of tetracyclines and so should be avoided within 2-3 hours of taking these antibacterial drugs.

ACE inhibitors and Potassium-Elevating Drugs / Supplements

ACE inhibitors increase the risk of hyperkalemia (elevated potassium levels); and so this risk is increased when they are taken with potassium-elevating drugs or potassium supplements.

First-Generation Antihistamines and Drugs with Sedative Effects

First-generation antihistamines can cause sedation (such as chlorphenamine). This risk is increased when taken with other drugs or substances that also cause sedation.

Antipsychotic Drugs and Drugs that Prolong the QT Interval

Antipsychotic drugs can prolong the QT interval, meaning that they can increase the risk of cardiac arrhythmias.

Aspirin and Antiplatelet or Anticoagulant Drugs

Aspirin at low doses is used for its antiplatelet effects. Therefore, the risk of bleeding increases when aspirin is taken with antiplatelet or anticoagulant drugs.

Opioids and Benzodiazepines

Opioids can cause respiratory depression. This risk increases when taken with other drugs that affect the CNS, such as benzodiazepines.

Beta-2 agonists and Beta-blockers

The effects of beta-2 agonists are opposed by beta-blockers (such as labetalol).

Beta-blockers and Calcium Channel Blockers

Beta-blockers must be avoided with certain calcium channel blockers (verapamil and diltiazem) because the combination can cause heart failure and very low heart rate (bradycardia).

Bisphosphonates and Drugs or Supplements with Multivalent Ions

Multivalent ions interact with and reduce the effect of bisphosphonate drugs.

Anticonvulsant Drugs and Drugs that Lower the Seizure Threshold

Some drugs lower the seizure threshold and reduce the efficacy of anticonvulsant drugs.

Antimicrobial Drugs and Warfarin

Antimicrobial drugs can enhance the bleeding risk of warfarin.

Corticosteroids and NSAIDs

Systemic corticosteroids increase the risk of peptic ulceration in patients taking NSAIDs.

Corticosteroids and Beta-2 Agonists / Loop-Thiazide Diuretics

Systemic corticosteroids increase the risk of low potassium levels (hypokalemia) in patients taking either beta-2 agonists or loop or thiazide diuretics.

Loop Diuretics and Aminoglycosides

Both loop diuretics and aminoglycosides increase the risk of kidney and ear toxicity.

Dopaminergic Drugs and Antipsychotic Drugs

Antipsychotic drugs block dopamine receptors and oppose the effects of dopaminergic drugs.

Macrolide Antibacterials and Statins

Macrolides increase the risk of muscle-related side effects of statins.

Metformin and IV Contrast Media

There is an increased risk of kidney damage when metformin is taken alongside IV contrast media.

Methotrexate and Penicillins

Penicillins reduce the excretion of methotrexate and so increase the risk of toxicity.

PDE5 Inhibitors and Nitrates

Both PDE5 inhibitors and nitrates increase the risk of hypotension.

Phenytoin and Estrogens / Progestogens

Phenytoin reduces both the concentration and efficacy of medicines that contain either estrogens or progestogens.

Quinine and Drugs that Prolong the QT Interval

Quinine can prolong the QT interval, increasing the risk of arrhythmias.

Fluoroquinolones and Prednisolone

When taken with prednisolone, fluoroquinolone antibacterial drugs increase the risk of tendon rupture.

Trimethoprim and Potassium-Elevating Drugs

As trimethoprim can also cause hyperkalemia, the combination of drugs must be avoided.

Immunosuppressant Drugs and Live Vaccines

Immunosuppressant drugs reduce the immune response needed when live vaccines are administered.

Vancomycin and Aminoglycosides or Loop Diuretics

The risk of ototoxicity and nephrotoxicity increases when vancomycin is taken with either an aminoglycoside or a loop diuretic.

Z-drugs and Alcohol

Alcohol significantly increases the risk of CNS effects such as drowsiness and respiratory depression when taken with Z-drugs.

Allopurinol and Amoxicillin

Co-prescription of allopurinol with amoxicillin increases the risk of skin rash.

SSRI Antidepressants and Aspirin or NSAIDs

SSRI increase the risk of GI bleeding, worsened when taken with aspirin or NSAIDs.

MAO Inhibitors and Tyramine

There is a risk of hypertensive crisis if MAO inhibitors are taken with tyramine-rich foods.

5-HT3 Antagonists and Drugs that Prolong the QT Interval

5-HT3 antagonists can prolong the QT interval and must be avoided with other drugs that also cause this effect.

Levothyroxine and Insulin

Levothyroxine can affect insulin requirements in patients with diabetes mellitus.

Rifampicin and Oral Contraceptives

Rifampicin can reduce the efficacy of hormonal contraception.

Metronidazole and Warfarin

Metronidazole reduces the metabolism of warfarin, increasing the risk of bleeding.

Methotrexate and Clozapine

There is a risk of dangerously low white blood cell count if methotrexate and clozapine are taken at the same time.

Lithium and Loop Diuretics

Loop diuretics can significantly increase levels of lithium.

Heparins and Warfarin or Antiplatelet Drugs

Heparins increase the risk of bleeding when taken with warfarin or other antiplatelet drugs.