Concomitant drugs refer to two or more drugs or medications taken at the same time. The time frame that constitutes the same time varies according to desired effect of the drug, the half-life of each drug administered, the metabolism of each drug, the excretion of each drug and the side effects of each drug individually and collectively. For instance, chemotherapy is often cited as a prime example of therapeutic concomitant drugs whereby two or more medications are administered literally at the same time for the sake of the patient's health. Yet, another example of concomitant drugs could be tetracycline antibiotics taken with supplemental calcium, where the calcium is taken just once in the morning and the antibiotic is administered every 6 hours until completed. In addition showing how in this case, "the same time" means "same day," this example illustrates one of the negative effects of drugs taken concomitantly — side effects that occur in combination — as concomitant calcium use decreases the effectiveness of the antibiotics.
Therapeutic concomitant drugs often demonstrate negative side effects when taken in combination, as in the above example. For this reason, doctors often ask patients what medications they are regularly taking with every office appointment, and pharmacists carefully monitor patients' drug regimes. Pharmaceutical companies continue to collect data on reported side effects of the medications they manufacture long after approval testing is complete. An integral aspect of the information they continue to collect is the other drugs a patient takes in addition to their own medication. Over time, this information can reveal combined side effects from concomitant drugs.
In addition to producing side effects when combined, concomitant drugs can also block or slow another drug's metabolism by interfering with the digestive pathway or excretion by the kidneys or the liver, thereby markedly increasing the blocked drug's half-life. If the blocked drug remains in its active form, symptoms of an overdose of that medication may develop, despite the patient having taken the drugs exactly as ordered. Alternately, a blocked drug may remain unable to reach its potent active metabolite and prove ineffective for the patient at a therapeutically prescribed and administered dosage.
Concomitant drugs is also a term recognized with drug abuse or recreational use of street drugs. This type of concomitant use can be used to potentiate the desired effect of the medications, such as in the use of tranquilizers with sedatives. Conversely, street and recreational drugs are also often combined in opposition to the primary effect of each — so as to minimize the unpleasant or undesired side effects — such as the concomitant use of cocaine and alcohol. The most common drug combined with others in this type of concomitant drug abuse is alcohol.