Drug Contraindications
- Trisha Y
- May 1, 2024
- 3 min read
What is a drug contraindication and why are they important?
The human body is inherently all-connected and it must maintain a delicate balance for all the parts of the body to work. A drug contraindication is any factor that makes it dangerous to receive a certain drug. The factors can include medical conditions, diet, and other drugs. Drug contraindications are important because they affect how doctors pick a treatment for a patient.
Drug-Drug Contraindications:
Drug-drug contraindications, also known as drug-drug interactions, are the interactions between two drugs used on one person, possibly causing harm. One drug can slow, decrease, increase the absorbance rate from the other drug, or the effects of the two can overlap. This is dangerous due to dosing- too much of a certain drug can cause organ damage (often the liver or kidneys), and too little can be ineffective and not treat what it’s prescribed for. These categories are known as duplication (intensified side effects), opposition (opposing actions), and alteration (alters how the body distributes or metabolizes the drug). These interactions can happen with any two drugs, but doctors are trained to understand how drugs interact and prescribe safely. Extra consideration must be taken with over-the-counter drugs due to their seemingly harmless effects.
Examples of drug-drug interactions:
One notable drug interaction is between ondansetron and dofetilide. Ondansetron is an antiemetic that works by blocking the action of serotonin 5-HT3. Serotonin 5-HT3 is shown to decrease heart rate. Dofetilide also decreases heart rate, meaning that the patient’s heart rate could drop to dangerous levels. This interaction comes from the effects of both the drugs overlapping.
Another drug interaction is between a blood thinner and a medication with a side effect of bleeding. Some antibiotics, like sulfamethoxazole, can raise the risk of bleeding. When taken with blood thinners (such as warfarin), there is a risk of serious bleeding. This interaction comes from the effects of the drug enabling each other.
An example of a drug interaction that slows down the absorption of each other is levothyroxine (a thyroid medication) and omeprazole (a heartburn medication). Omeprazole can lower the absorption rate of levothyroxine, leading to the levothyroxine being ineffective.
Drug-condition interactions:
Drug-condition interactions are interactions that occur when a drug’s effects can interact with a different condition the patient has. These interactions can worsen the condition, reduce the drug efficacy, or both. This is dangerous especially if the condition is life-threatening or the drug is needed for survival.
Examples of drug-condition interactions:
An example of worsening the condition is propranolol, a non-selective beta blocker that treats high blood pressure. This medicine can cause bronchospasm, where the muscles along the bronchi (that help you breathe) tighten up. This is a problem when the patient has conditions that inhibit breathing, such as asthma or lung cancer. This contraindication also occurs with other non-selective beta blockers, but propranolol is the most common.
An example of lowering the efficacy of the drug is furosemide and severe kidney disease. Furosemide is a diuretic that treats high blood pressure by treating edema (fluid build-up). Diuretics work by causing the kidneys to release unneeded water and salt from the body into the urine. With severe kidney disease, the kidneys won’t work as well. This means the furosemide will be less effective, and the edema will not be treated as much as the dose allows.
Drug-food interactions:
Drugs and food can interact in various ways. The food could increase or lower the concentration levels of drugs in the blood. Certain foods interact with drugs more often than others, such as grapefruit.
Grapefruit interacts with many drugs in different ways due to how it can block the action of intestinal CYP3A4, which is a vital enzyme for breaking down and metabolizing drugs. This increases blood concentration because the drug enters the blood instead of being metabolized. For example, grapefruit increases the concentration of some statins in the blood. Statins lower the cholesterol in the blood, but too much statin concentration in the blood can lead to liver damage. On the opposite side of the spectrum, grapefruit can also lower the absorption of drugs. Grapefruit can affect proteins in the body called drug transporters that move the drug into our cells for absorption. An example is fexofenadine (an antihistamine), where the grapefruit prevents the drug from being absorbed, which can lead to the antihistamine not taking effect.
Drug contraindications are an area of patient care that must always be considered, especially with high risk patients. If not considered, the drug could cause many harmful effects, sometimes even worse than what it is treating.
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