Additional information and drug contraindications
Common drug interactions
| Drug-drug combination | Effect |
|---|---|
| ACE inhibitors + potassium supplements | Hyperkalemia |
| ACE inhibitors + spironolactone | Hyperkalemia |
| Digoxin + amiodarone | Digoxin toxicity |
| Digoxin + verapamil | Bradycardia, heart block, digoxin toxicity |
| Theophylline + quinolones | Theophylline toxicity, seizures |
| Warfarin + macrolides/quinolones/sulfa antibiotics | Bleeding |
| Warfarin + NSAIDs | Gastrointestinal bleeding |
| Warfarin + phenytoin | Warfarin toxicity, phenytoin toxicity |
| Simvastatin + fluconazole | Increased simvastatin levels, liver toxicity |
| Levothyroxine + omeprazole | Decreased absorption of levothyroxine |
| Metronidazole + alcohol | Nausea, vomiting, flushing, headache |
Drug contraindications
A contraindication is a specific situation where a medicine, procedure, or surgery shouldn’t be used because it may harm the person.
A drug may be contraindicated because of:
- An underlying disease (for example, liver or kidney failure)
- A patient factor (for example, pregnancy or older age)
- A severe interaction with another medication
There are two types of contraindications:
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Relative contraindication: The medication carries a significant risk of harm, but it may still be used when the expected benefit outweighs the risk. In these cases, risk-reducing steps are used (for example, closer monitoring of drug levels or adverse effects). For example, warfarin is not combined with aspirin because it may cause bleeding.
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Absolute contraindication: The medication could cause a life-threatening situation, so it must be avoided. Examples include isotretinoin, which is contraindicated in pregnancy due to the high risk of birth defects; sildenafil, which is contraindicated with nitrates due to severe hypotension; cisapride, which is contraindicated with multiple drugs, including macrolides like erythromycin, antifungals like fluconazole, and protease inhibitors like ritonavir; and MAO inhibitors, which are contraindicated with SSRIs.
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Drugs contraindicated in pregnancy: Many medications can cause birth defects and fetal loss if used during pregnancy, especially during the first trimester. They include isotretinoin, blood pressure medications like ACE inhibitors and ARBs, antiepileptics like sodium valproate and valproic acid, phenytoin, carbamazepine, phenobarbital and topiramate, benzodiazepines, thalidomide, oral contraceptives, warfarin and ibuprofen. Drugs classified as FDA “Category X” are contraindicated in pregnancy because studies have demonstrated toxic effects on the fetus.
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Drugs contraindicated in old age: In older age, organs like the liver and kidneys may not function optimally. This increases the risk of adverse effects because these organs are major sites of drug metabolism and excretion. Age-related changes in body fat, water, and muscle mass can also affect how drugs are distributed in the body. Beer’s criteria is used to determine the safety of a medication in the elderly. Such medications may be OTC or prescription medications, for example antipsychotics like haloperidol and risperidone, anticholinergics like amitriptyline, imipramine, dicyclomine and trihexyphenidyl, sleeping pills like zolpidem and zaleplon, benzodiazepines, muscle relaxants, and OTC products containing diphenhydramine (Benadryl) and chlorpheniramine. Blood thinners like aspirin and antidiabetics like glyburide and glipizide must be used with caution because the risk of adverse effects like bleeding and hypoglycemia can be very high.