What Is Drug Toxicity?

Toxicity refers to how poisonous or harmful a substance can be. In pharmacology, drug toxicity occurs when a person has accumulated too much of a prescription drug in their bloodstream, leading to negative effects.

Drug toxicity is formally defined as "a diverse array of adverse effects which are brought about through drug use at either therapeutic or non-therapeutic doses." Learn how drug toxicity occurs, the signs it may exist, and how drug toxicity is treated.

Causes of Drug Toxicity

Drug toxicity can occur as a result of the over-ingestion of medication, causing too much of the drug to be in a person's system at once. This can happen if the dose taken exceeds the prescribed amount, or if the prescribed dosage is too high.

With certain medications, drug toxicity can also occur as an adverse drug reaction. In this case, the normal therapeutic dose of the drug can cause unintentional, harmful, and unwanted side effects.

In some cases, the threshold between an effective dose and a toxic dose is very narrow. A therapeutic dose for one person might be toxic to another person. Plus, drugs with a longer half-life can build up in a person's bloodstream and increase over time, resulting in drug toxicity.

Additionally, factors such as age, kidney function, and hydration can affect how quickly your body is able to clear a medication from your system. This is why medications such as lithium require frequent blood testing to track the levels in the bloodstream.

Symptoms of Drug Toxicity

How do you know if you or someone you know is experiencing drug toxicity effects? Drug toxicity symptoms can differ depending on the medication you are taking. In the case of lithium, for instance, mild symptoms of acute lithium toxicity (which is drug toxicity after taking the drug one time) can include:

• Diarrhea
• Dizziness
• Nausea
• Stomach pains
• Vomiting
• Weakness

More severe symptoms of acute lithium toxicity may appear in the form of:

• Ataxia (poor muscle control, resulting in clumsy movements)
• Coma
• Hand tremors
• Heart problems (in rare cases)
• Muscle twitches
• Nystagmus (involuntary jerking of the eyeball)
• Seizures
• Slurred speech

Tegretol (carbamazepine) is a drug often prescribed for epilepsy but also used to help treat mental health conditions such as schizophrenia and bipolar disorder. Some patients with carbamazepine toxicity have experienced symptoms including:

• Blurred vision
• Dizziness
• Nystagmus
• Trouble walking

In cases of severe carbamazepine toxicity, symptoms can include:

• Altered levels of consciousness
• Coma
• Seizures
  

Diagnosis of Drug Toxicity

Acute drug toxicity is more easily diagnosed as the symptoms follow the taking of the medication just one time. Blood tests can also screen for levels of the medication in the bloodstream, showing whether these levels are too high.

Chronic drug toxicity, or drug toxicity that occurs due to long-term build-up, is harder to identify. Stopping the medication, then "re-challenging" it later is one method of testing whether the symptoms are caused by the medicine. This method can be problematic, however, if the medication is essential and doesn't have an equivalent substitute.

Treatment of Drug Toxicity

There are several ways drug toxicity may be treated. If the toxicity results from an acute overdose, a person may undergo stomach pumping to remove drugs that have not yet been absorbed. 

Activated charcoal is another drug toxicity treatment option. It can be used to bind the drug, preventing it from being absorbed into the blood. (If this approach is used, the drug is eliminated from the body through stool). Other medications may also be given as an antidote for drug toxicity.