Terms in Pharmacology, Part 2: Drug Interactions, Tolerance, Dependence and More
8 minute read
What you will learn
- Define antagonism, synergism, additive effect and potentiation
- Explain the cumulative effect
- Define tolerance and dependence
- Distinguish addiction, abuse and misuse
- Define chemotherapy, therapeutics and placebo
- Give an example of each term
Key points
- Antagonism: one drug reduces or blocks the effect of another.
- Additive effect: the combined effect equals the sum of the separate effects. Synergism: the combined effect is greater than the sum.
- Potentiation: one drug increases the effect of another that alone has little or no effect.
- Cumulative effect: a drug builds up in the body when it is given faster than it is removed.
- Tolerance: after repeated use, a larger dose is needed to get the same effect.
- Dependence is the need for a drug to feel normal. Addiction is compulsive drug seeking despite harm.
- Misuse is using a medicine wrongly. Abuse is using a drug for non-medical reasons, or in excess.
When drugs act together
When two or more drugs are taken together, their effects may combine in different ways. Using numbers as a memory aid:
| Term | Meaning | Memory aid | Example |
|---|---|---|---|
| Additive effect | The combined effect equals the sum of the separate effects | 1 + 1 = 2 | Two painkillers of different kinds taken together, as advised |
| Synergism | The combined effect is greater than the sum | 1 + 1 = 3 | Sulfamethoxazole with trimethoprim (co-trimoxazole) |
| Potentiation | One drug, with little or no effect alone, increases the effect of another | 0 + 1 = 2 | Probenecid raises the blood level of penicillin, so penicillin acts for longer |
| Antagonism | One drug reduces or blocks the effect of another | 1 + 1 = 0 | Naloxone blocking morphine |
Some interactions are useful and some are dangerous. The pharmacist checks every prescription for them, and a technician reports anything unusual, such as a patient on several medicines or buying over-the-counter products that may interact.
Types of antagonism
- Pharmacological: both drugs act on the same receptor, as with naloxone and morphine.
- Physiological: the drugs have opposite effects through different routes.
- Chemical: the drugs react with each other, as in using an antidote to bind a poison.
Cumulative effect
A drug has a cumulative effect when it builds up in the body, because each new dose is given before the earlier one has been removed. Eventually it may reach toxic levels. This is a risk with drugs that have a long half-life and in patients with poor kidney or liver function.
Tolerance
Tolerance means that, after repeated use, a larger dose is needed to produce the same effect. The body adapts to the drug. It is common with opioid painkillers, sedatives and alcohol.
- Tolerance can develop quickly, in a few doses, as with some drugs (this is called tachyphylaxis), or slowly, over weeks or months.
- Tolerance can be dangerous, since patients may take more than is safe.
- Cross-tolerance occurs between similar drugs.
Dependence and addiction
| Term | Meaning |
|---|---|
| Physical dependence | The body has adapted to the drug, and stopping it causes withdrawal symptoms, such as shaking, sweating, pain or anxiety |
| Psychological dependence | A strong mental need or craving for the drug, to feel normal or to cope |
| Addiction | Compulsive drug seeking and use, despite harm to health, work, family or finances |
| Withdrawal | The unpleasant symptoms when a drug that the body depends on is stopped suddenly |
Drugs that may cause dependence include opioids, sedatives (benzodiazepines), stimulants and alcohol. Addiction is a health problem, and people with addiction need help and treatment, not blame. See the lesson on dependence in AUM 321.
Abuse and misuse
| Term | Meaning | Example |
|---|---|---|
| Drug misuse | Using a medicine wrongly, not as directed | Taking more paracetamol than advised, sharing antibiotics, stopping a course early |
| Drug abuse | Using a drug for non-medical reasons, or in excess, in a way that harms the user or others | Taking a strong painkiller or a sedative to get high |
A pharmacy technician should notice warning signs, such as repeated requests for the same medicine, and tell the pharmacist. See the lessons on ethical dispensing in BDP 332.
Other terms
| Term | Meaning |
|---|---|
| Chemotherapy | The use of drugs to kill or stop the growth of germs, parasites or cancer cells, with little harm to the patient. Often used for infections and cancer |
| Therapeutics | The branch of medicine that deals with the treatment of disease |
| Placebo | An inactive substance given in place of an active drug, often to test a new medicine, and which may help because of the patient's belief |
| Drug interaction | A change in the effect of a drug because of another drug, food or herb |
| Antidote | A substance that counteracts a poison or overdose |
Quick comparison to remember
| If you see… | Think of… |
|---|---|
| Needs more drug for the same effect | Tolerance |
| Cannot stop without symptoms | Dependence |
| Compulsive use despite harm | Addiction |
| Drug blocks another | Antagonism |
| Combined effect larger than the sum | Synergism |
| Drug with no effect alone boosts another | Potentiation |
| Builds up in the body | Cumulative effect |
Exam focus
- Define antagonism, synergism and potentiation, and give one example of each.
- State the difference between tolerance and dependence.
- State the difference between abuse and misuse.
- Common mistake: using synergism and potentiation as if they meant the same.
Quick check
Test yourself before you move on. Pick an answer to see the explanation.
1.Naloxone reverses the effects of morphine. This is an example of:
2.Two drugs given together produce an effect greater than the sum of their separate effects. This is:
3.A patient who has used a painkiller for a long time now needs a larger dose for the same relief. This is:
4.Using a sedative for 'kicks' rather than for illness is best described as:
5.A placebo is: