How Many is Too Many?


A drug interaction is a situation in which a substance (usually another drug) affects the activity of a drug when both are administered together. This action can be synergistic (when the drug’s effect is increased) or antagonistic (when the drug’s effect is decreased) or a new effect can be produced that neither produces on its own. Typically, interactions between drugs come to mind (drug-drug interaction). However, interactions may also exist between drugs and foods (drug-food interactions), as well as drugs and medicinal plants or herbs (drug-plant interactions).

When a person uses a drug regularly they develop tolerance to it. This means they need to use more to get the same effect. Similarly, if a person hasn’t been using regularly – or if they’ve not been able to get drugs – their tolerance will drop. When people take their usual amount of drugs after a break from using, it could be too much for the body to cope with and lead to an overdose. This is why high-risk situations for drug overdose include post-release from prison, detoxification and rehabilitation. Someone on naltrexone can also be at risk if they use soon after stopping oral medication, or skipped a dose, or when the effects of a naltrexone implant have ceased.Pills photo by dimshik

“Half-life” refers to the time it takes for a drug to drop to half the strength of its original dose. Some drugs have a long half-life, for example some benzodiazepines. If a person has used yesterday, they may still have enough in their system today to overdose if they use more. Diazepam (Valium) has one of about 24 hours, so if you took 20mg yesterday you would still have approximately 10mg of diazepam active in your system today. If you were then to use heroin or morphine, you would have an increased risk of overdose as you would be using the opioids in addition to that 10mg of diazepam.

That is only one of the reasons why taking many drugs, prescription or otherwise, is dangerous - especially at the same time. Drug interactions, or pharmacological incompatibilities, occur when two or more drugs or chemicals are mixed outside the body for the purpose of joint administration (using at the same time). Certain drugs, for example, cannot be administered in plastic bottles because they bind with the bottle’s walls and have a reaction with the chemicals in the plastic.

It is possible to take advantage of positive drug interactions. However, the negative interactions are usually of more interest because of the significance of their effects and also because they are often unexpected and may even go undiagnosed. There are a few things that can tip doctors, or friends/family that something is going wrong - here are a few:

  • Old age: factors relating to how human physiology changes with age may affect the interaction of drugs. For example, liver metabolism, kidney function, nerve transmission or the functioning of bone marrow all decrease with age. In addition, in old age there is a sensory decrease that increases the chances of errors being made in the administration of drugs.
  • Polypharmacy: The more drugs a patient takes the more likely it will be that some of them will interact.
  • Genetic factors: Genes create enzymes that make drugs react. Some human races have variations in their genes that could decrease or increase the activity of these enzymes. The consequence of this would, on occasions, be a greater predisposition towards drug interactions and therefore a greater predisposition for adverse affects to occur. This is seen in genotype variations in the isozymes of cytochrome P450 (an enzyme).
  • Hepatic or renal diseases: The blood concentrations of drugs that are metabolized in the liver and / or eliminated by the kidneys may be altered if either of these organs is not functioning correctly. If this is the case an increase in blood concentration is normally seen.
  • Drug dependent factors:
    • Narrow therapeutic index : Where the difference between the effective dose and the toxic dose is small.
    • Steep dose-response curve: Small changes in the dosage of a drug produce large changes in the drug's concentration in the patient’s blood plasma.
    • Saturable hepatic metabolism: In addition to dose effects, the capacity to metabolize the drug is greatly decreased