Dangerous Mix


Avoid these common drug interactions

Drug interactions occur when the effectiveness of a medicine is reduced, or its toxicity is increased by another drug, disease, condition or food. There are drug-drug interactions, drug-disease interactions and drug-food interactions. All drugs have a potential for interaction with other drugs but other factors such as the number of medications one is taking, the age of the patient, the disease, the condition of the kidney and liver, might increase the risk and severity of drug interactions. For example, people with liver and kidney disease have reduced capacity to metabolise and excrete drugs respectively, and if they took drugs that interact, the effects may be more severe than those with normal functioning organs.

Drug-food Interactions

When some foods are taken with medication, they can interfere with the function of the medicine. A good example is warfarin, an anticoagulant used in treatment and prevention of clotting disorders. Foods rich in Vitamin K like kale (sukuma wiki) and other green leafy vegetables should be avoided or taken in minimal quantities in patients taking warfarin because they reduce its effectiveness in management of clots. Warfarin works by inhibiting Vitamin K dependent clotting factors in the liver. High intake of Vitamin K reduces the effectiveness of warfarin.

People suffering from migraines should avoid red wine, chocolates, cheese and other tyramine-rich foods as they tend to precipitate migraines and worsen their occurrence.

Grapefruit juice inhibits an enzyme called cytochrome P450 which metabolises several drugs such as valium, xanax, amphetamines and antiretrovirals such as ritonavir among other drugs, and this causes their accumulation and toxicity.

Drug-disease Interactions

There are many drug-disease interactions. But to mention a few, diseases of the liver reduce its capacity to metabolise drugs. The drugs then accumulate and become toxic. Commonly used drugs like paracetamol can be toxic in liver disease. (Side note: Taking paracetamol after binge drinking potentiates the toxic effects of alcohol in the liver. It is advisable to take another pain killer for hangover headaches!).

The kidney is the major organ that excretes drugs from our bodies. Drugs that are toxic to the kidney become more toxic when the kidney is failing. Those that are mainly excreted by the kidney accumulate and become toxic. Antibiotics like gentamicin and ceftriaxone should be avoided in patients with severe kidney disease and renal failure.

Drug-drug Interactions

Drug- drug interactions can be mild or life threatening. There are several mechanisms through which drug interactions can occur. The cytochrome P450 enzymes in the liver are responsible for metabolising many drugs in the body. Some medicines induce (or potentiate) the function of these enzymes. Such drugs include those used in the treatment of epilepsy and other seizure disorders. Anti-TB drug Rifampicin is also an inducer of cytochrome P450. When these inducers are taken with drugs that are metabolised by these enzymes such as oral contraceptives, they cause reduction in their effectiveness, causing unwanted pregnancy.

The drugs that inhibit the functions of these enzymes reduce the capacity of the enzymes to metabolise other drugs. They accumulate and become toxic. Amiodarone is a medicine used in the treatment of heart rhythm disorders. When this medicine is taken together with warfarin, an anticoagulant, it causes accumulation of warfarin and this leads to toxicity, which manifests as bleeding.

Clarithromycin and erythromycin are common antibiotics. When taken together with verapamil, a medicine used for several heart conditions and hypertension, they cause reduction in the metabolism of verapamil, causing hypotension and heart rhythm disorders.

Drug interactions can also occur independent of the Cytochrome P450 enzyme system. For example, when broad spectrum antibiotics like tetracycline are taken together with oral contraceptives, they kill the bacteria in the digestive system that are responsible for breaking down oestrogen to its active form. This leads to reduced effectiveness of the contraceptives and unplanned pregnancies.

Fluoxetine (Prozac) is commonly used in treatment of depression. It is classified as a selective serotonin reuptake inhibitor. When taken with other drugs that increase serotonin levels in the brain, such as drugs of the monoamine oxidase inhibitor class like phenelzine, it causes a life threatening condition called serotonin syndrome. Patients can have seizures, increased brain activity, increased heart rate, profuse sweating, mental status changes and death. It is advised that fluoxetine be stopped at least five weeks before a monoamine oxidase inhibitor is prescribed.

Digoxin is an important drug used in heart failure. Quinidine is used for treatment of heart rhythm disorders. When taken together, levels of digoxin increase to toxic levels. Digoxin is one of the drugs described as having a narrow therapeutic index. This means that there is a very small difference in safe treatment levels and toxic levels. Digoxin toxicity leads to impairment in heart rhythm and death.

Isosorbide mononitrate is a medicine used in the treatment of heart conditions such as angina. When people taking this medicine take Viagra, Viagra increases the hypotensive effects of isosorbide mononitrate and this can lead to death.

Methotrexate used in cancers and immune disorders like arthritis, when taken with medicines like probenecid and antibiotics like amoxicillin and salicylates like aspirin, these drugs reduce the excretion of methotrexate leading to accumulation and toxicity. Toxicity may manifest as diarrhoea, vomiting, excessive sweating and kidney failure.

Bromocriptine used in treatment of Parkinson’s disease when taken with cough syrups containing pseudoephedrine, a life threatening interaction is brought about with seizures and increased heart rates which lead to death.

Note: Since drug interactions can occur with any medicine, food, and disease, it is important that anyone taking medicines consults the pharmacist on the medicines that should not be taken together. They can also advise on the different times to take scheduled medicines if they must be taken together. Insist on speaking to the pharmacist to get a full medication consultation and not the pharmacy techs at the counter. It is also advisable that when one is purchasing a medicine whether over-the-counter or using a prescription, they should inform the pharmacist which other medication they are taking and if possible, bring them over from home.

Dr Sylvia Opanga is a Pharmacotherapy Specialist at Kenyatta National Hospital and Lecturer at The School of Pharmacy, University of Nairobi.

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