Drug Interactions

Pharmacodynamics and pharmacokinetics

When an individual takes drugs, not only does the substance affect him but he himself influences the substance. The duration and type of effect of a drug, pharmacodynamics, is related to the change in its concentration in the body over time, the pharmacokinetics.

This concentration can be heavily influenced by the individual consuming it (individual variability) as well as by the presence of other substances in the bloodstream.

Such drug interactions can either increase or decrease the expected effect of a substance. In some cases these interactions are planned (replacement therapy), in others they are sought to increase the effect. In many cases, however, they are not planned or expected at all, and the adverse consequences are often disastrous.

Interactions can be predictable, repeatable, or be “random” to occur only in isolated cases.

Amphetamines and drug interactions

Substances that acidify (e.g. raspberry juice) or alkalize (e.g. bicarbonate) the urine, i.e. substances that modify its pH, affect the pharmacokinetics of amphetamines due to the fact that their elimination rate is higher when the urine is acidic and lower when they are basic. This procedure is intentional and is generally used to influence analyzes.

LSD + antidepressants

It has been repeatedly reported that the LSD “flashback” phenomenon among adolescents is easily established when they receive antidepressant therapy based on serotonin “reuptake” inhibitors (eg paroxetine).

Chronic use of tricyclic antidepressants (eg Prozac) is reported to increase physical, psychological and hallucinatory responses to LSD.

Cannabis + Alcohol

Controlled studies report that the combined use of these substances causes a degree of “disability” higher than that caused by the individual substances.

However, it has been noted that this “disability” disappears the day after administration.

Cocaine + Alcohol

This type of interaction is almost unique. The combination of these two substances leads to the formation in the body of a new substance called coca ethylene. This compound is formed only when the two substances are used together. Although the rate of elimination of this compound is known to be slower than that of related substances, its importance remains unclear except for analytical purposes.

Opiate analgesics

Antibiotics are frequently used in patients undergoing medical or surgical procedures and very often antibiotics interfere with the action of opiates used for analgesic treatment.

The best known interactions are those with erythromycin (which produces an increase in the action of opiates) and rifampicin (a decrease).

Another interaction occurs with drugs used in the treatment of epilepsy, in particular carbamazepine, phenytoin and barbiturates which increase the rate of metabolism of opiates in the liver.

The tricyclic antidepressants clomipramine and amitriptyline also significantly increase the plasma availability of morphine.

One of the more “violent” opioid interaction effects is seen in the case of “rapid detoxification” when a small dose of naloxone (0.5 mg) is injected into drug addicted patients. This interaction precipitates a withdrawal syndrome very similar to that seen for not taking opiates, with the difference that it appears in a few minutes and passes in a couple of hours.

Drugs with depressive action and drug interactions

The combination of such drugs as alcohol and heroin or alcohol and tranquilizers is one of the most common types of interactions.

And it is also one of the most dangerous because the effect of one enhances the effect of the other.

Alcohol is probably the most dangerous “mixer”. In fact, there are many deaths from heroin overdose that also involve the simultaneous presence of alcohol. Similarly, pharmacological doses of barbiturates can easily become lethal if associated with alcohol.