Drug Interactions

DRUG INTERACTIONS

Drug interaction refers to modification of response to one drug by another when they are administered simultaneously or in quick succession. The modification is mostly quantitative, i.e. the response is either increased or decreased in intensity, but sometimes it is qualitative, i.e. an abnormal or a different type of response is produced. The possibility of drug interaction arises whenever a patient concurrently receives more than one drug, and the chances increase with the number of drugs taken.

Many medical conditions are treated with a combination of drugs. The components of the combination are so selected that they complement each other’s action, e.g. an antibiotic is used along with an analgesic to treat a painful infective condition; adrenaline is combined with lidocaine for local anaesthesia; antitubercular drugs are combined to prevent drug resistance; mixed aerobic-anaerobic bacterial infections, are treated with a combination of antimicrobials. More commonly, multiple drugs are used to treat a patient who is suffering from two or more diseases at the same time. The chances of unintended/adverse drug interactions are greater in this later situation, because an assortment of different drugs may be administered to a patient depending on his/her diseases/symptoms.

Several drug interactions are desirable and deliberately employed in therapeutics, e.g. the synergistic action of ACE inhibitors + diuretics to treat hypertension or sulfamethoxazole + trimethoprim to treat bacterial infection or furosemide + amiloride to prevent hypokalaemia. These are well-recognized interactions and do not pose any undue risk to the patient. The focus of attention in this chapter are drug interactions which may interfere with the therapeutic outcome or be responsible for adverse effects, or may even be fatal (bleeding due to excessive anticoagulant action).

The severity of drug interactions in most cases is highly unpredictable. However the doctor must know which drugs are not to be prescribed concurrently. More importantly, a large section of patients may be receiving one or several drugs for their chronic medical conditions like hypertension, diabetes, arthritis, etc. (see below for regular medication drug classes employed commonly). The physician may prescribe certain drugs which may interact with those already being taken by the patient and result in adverse consequences. It is, therefore, imperative for the doctor to elicit a detailed drug history of the patient and record all the medication that he/she is currently on. The list of potential adverse drug interactions is already quite long and constantly growing. It is practically impossible for anyone to know/remember all possible drug interactions. Fortunately, the clinically important and common drug interactions that may be encountered in routine practice are relatively few. Some of these are listed in Table 69.1. More exhaustive compilations and documentation are available in specialized books, monographs, review articles and computer database on the subject, but these also need constant updating.

Regular Medication Drugs

(Likely to be involved in drug interactions)

Antidiabetics

Antihypertensives

Antianginal drugs

Antiarthritic drugs

Antiepileptic drugs

Antiparkinsonian  drugs

Oral contraceptives

Anticoagulants

Antiasthmatic drugs

Psychopharmacological agents

Antipeptic ulcer drugs

Corticosteroids

Antitubercular drugs

Anti-HIV drugs

Certain types of drugs can be identified that are most likely to be involved in clinically important drug interactions. The physician may take special care and pay attention to the possibility of drug interactions when the patient is receiving one or more of such medications, or when the doctor intends to prescribe any of such drugs.

interactions

Types Of Drugs Most Likely To Be Involved In Clinically Important Drug Interactions

·      Drugs with narrow safety margin, e.g. aminoglycoside antibiotics, digoxin, lithium.

·     Drugs affecting closely regulated body functions, e.g. antihypertensives, antidiabetics, anticoagulants.

·      Highly plasma protein bound drugs like NSAIDs, oral anticoagulants, sulfonylureas.

·         Drugs metabolized by saturation kinetics, e.g. phenytoin, theophylline.