Discovery and development of drugs — AQA GCSE Biology
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Discovery and development of drugs explained
New medicines are developed through a sequence of stages.
Read the full explanation
Potential drugs may be discovered from traditional plant or microorganism sources, or synthesised in the laboratory. Preclinical testing uses cells, tissues and live animals to assess toxicity, dosage and efficacy before any human trial. Clinical testing then proceeds in phases: a small number of healthy volunteers checks safety and side effects; a larger group of patients checks effectiveness and optimum dose; and larger-scale trials compare the new drug with existing treatments, often using placebos and randomisation to reduce bias. If results show the drug is safe and effective, it can be licensed and marketed. Students should describe this process in order and explain why each stage is needed.
Traditionally drugs were extracted from plants and microorganisms.
For much of human history, medicines came from natural sources. Plants produce many chemicals that can have medicinal effects, and microorganisms such as moulds and bacteria produce substances that can kill pathogens or alter body processes. Traditional knowledge often identified these sources: for example, willow bark was used for pain and fever, and foxglove for heart conditions. Today, scientists still screen plants and microorganisms for useful compounds, and many modern drugs are either extracted from these sources or modified from natural molecules. Students should understand that natural sources are the origin of many drugs, and that extraction or chemical modification can produce effective medicines.
The heart drug digitalis originates from foxgloves. • The painkiller aspirin originates from willow. • Penicillin was discovered by Alexander Fleming from the Penicillium mould.
Many medicines began as natural substances. Digitalis, used to treat heart conditions, comes from foxgloves. Aspirin, a painkiller, comes from willow. Penicillin was discovered by Alexander Fleming when he noticed that Penicillium mould had killed bacteria growing on a culture plate. These examples show that drugs can be discovered by observing natural sources, then developed and tested before medical use. A student should be able to name each drug, its source and, for penicillin, the scientist and mould involved. They should also explain that discovery is only the first step: a promising substance must be purified, tested for safety and effectiveness, and produced in a suitable form before it can be prescribed.
Most new drugs are synthesised by chemists in the pharmaceutical industry. However, the starting point may still be a chemical extracted from a plant.
Drug discovery often begins with a natural chemical rather than a finished medicine. A plant may produce a compound that has a useful biological effect, such as relieving pain or killing pathogens. Scientists extract and identify this lead compound, then chemists in the pharmaceutical industry synthesise related molecules in the laboratory. Synthesis allows large, consistent quantities to be made and lets chemists modify the molecule to improve potency, reduce side effects or make administration easier. For example, the painkiller aspirin was developed from salicylic acid found in willow bark, but the medicine is now manufactured synthetically. So the starting point can be plant-derived, while most new drugs are made by chemical synthesis.
New medical drugs have to be tested and trialled before being used to check that they are safe and effective.
Before a new medicine can be prescribed, it must pass through testing and clinical trials. Testing checks two main things: safety, meaning the drug does not cause unacceptable harm, and efficacy, meaning it actually treats the condition. Preclinical laboratory work uses cells, tissues and live animals to look for toxicity and biological activity. If results are promising, clinical trials then involve healthy volunteers and patients. Trials are often randomised, controlled and sometimes blinded so that results are reliable. The drug is compared with a placebo or existing treatment, and data are analysed before a licence is granted. Without this evidence, doctors cannot know whether a drug will help more than it harms.
New drugs are extensively tested for toxicity, efficacy and dose.
Testing a new drug is not a single check but a detailed investigation of three linked properties. Toxicity testing asks whether the drug harms cells, tissues or whole organisms, and at what concentration harm appears. Efficacy testing asks whether the drug produces the intended therapeutic effect, such as lowering blood pressure or killing bacteria. Dose testing finds the amount that gives a useful effect with acceptable side effects, often by trying a range of doses. These three aspects interact: a drug that works only at a toxic dose is not suitable, and a safe drug that does nothing is useless. Extensive testing therefore builds a profile of benefits and risks before any wider use.
Preclinical testing is done in a laboratory using cells, tissues and live animals.
Preclinical testing happens before any testing in humans. It takes place in a laboratory and uses three main biological systems: cultured cells, isolated tissues and live animals. Cells can show whether a drug enters cells, affects cell division or causes cytotoxicity. Tissues can show effects on contraction, secretion or nerve impulses. Live animals allow the drug to be studied in a whole organism, where absorption, distribution, metabolism and excretion can be observed. Animal testing raises ethical issues, so it is regulated and carried out only when necessary. Results from preclinical testing help decide whether a drug is promising and safe enough to move into clinical trials with human volunteers.
Clinical trials use healthy volunteers and patients.
Clinical trials test a new drug in humans after laboratory and animal testing. Healthy volunteers are used first to check safety and how the body handles the drug, because they have no illness that could confuse results. Later, patients who have the target condition are used to see whether the drug actually works and whether benefits outweigh side effects. For example, a new asthma medicine might first be given to healthy adults at low dose, then to asthma patients to measure lung function. Using both groups lets researchers separate side effects caused by the drug from symptoms caused by the disease itself.
Very low doses of the drug are given at the start of the clinical trial. • If the drug is found to be safe, further clinical trials are carried out to find the optimum dose for the drug.
Clinical trials test a new drug in humans after laboratory and animal testing. They begin with very low doses to reduce the risk of harm, because the effects in humans are not yet fully known. Healthy volunteers or patients are monitored closely for safety and side effects. If the drug is found to be safe at these low doses, further clinical trials are carried out with more participants. These later trials increase the dose gradually to find the optimum dose: the dose that gives the best therapeutic effect with acceptable side effects. For example, a trial might start at 1 mg, then test 5 mg, 10 mg and 20 mg, recording how symptoms and side effects change. The optimum dose is not simply the highest dose; it balances benefit against harm. This staged approach protects participants and produces reliable evidence for licensing.
Your focus
- Describe the stages in the discovery and development of a new medicine.
- Explain the purpose of preclinical testing and each phase of clinical testing.
- Outline how placebos and randomisation are used to make clinical trials valid.
Show all 27 objectives
- Identify plants and microorganisms as traditional sources of drugs.
- Describe how active compounds are extracted from natural sources.
- Give examples of drugs that originate from plants or microorganisms.
- Name digitalis, aspirin and penicillin with their correct natural sources.
- Identify Alexander Fleming and the Penicillium mould in the discovery of penicillin.
- Describe why a discovered substance must be developed and tested before it is used as a medicine.
- Describe how a plant chemical can be the starting point for a new drug.
- Explain why pharmaceutical companies synthesise drugs rather than relying only on extraction.
- Use a named example to link a natural source to a synthesised medicine.
- Describe the purpose of testing and trialling new medical drugs.
- Explain how preclinical testing and clinical trials provide evidence of safety and efficacy.
- Justify why drugs must be licensed before being used to treat patients.
- Define toxicity, efficacy and dose in the context of drug testing.
- Describe how each property is investigated during drug development.
- Explain why a drug must be effective at a dose that is not unacceptably toxic.
- Describe the laboratory systems used in preclinical testing.
- Explain what each system can reveal about a new drug.
- Discuss why preclinical testing must be completed before clinical trials begin.
- State that clinical trials involve healthy volunteers and patients.
- Explain why healthy volunteers are used before patients.
- Distinguish safety testing from effectiveness testing in clinical trials.
- Describe why very low doses of a drug are given at the start of a clinical trial.
- Explain how further clinical trials are used to find the optimum dose after a drug is found to be safe.
- Distinguish between the optimum dose and the maximum dose in the context of clinical trials.
Discovery and development of drugs exam tips
Marking Points
- Discovery: potential drugs are identified from plant or microorganism extracts, or designed and synthesised in the laboratory.
- Preclinical testing: the drug is tested on cells, tissues and live animals to assess toxicity, safety and effective dose before human trials.
- Clinical testing phase 1: a small number of healthy volunteers is used to check safety and side effects.
- Clinical testing phase 2: a larger group of patients is used to test effectiveness and find the optimum dose.
- Clinical testing phase 3: large-scale trials compare the new drug with existing treatments, often using placebos and random allocation.
- If the drug is shown to be safe and effective, it is licensed and can be prescribed; otherwise development stops.
- Plants are a traditional source of drugs, with active compounds extracted from roots, leaves, bark or flowers.
- Microorganisms such as moulds and bacteria are also traditional sources of drugs, including antibiotics.
- Traditional or folk knowledge often guided which plants and microorganisms were investigated.
- Many modern medicines are derived from or based on natural compounds, even if later synthesised or modified.
- Extraction involves obtaining the active substance from the organism, sometimes followed by purification.
- Examples include digitalis from foxgloves and aspirin from willow, showing the plant origin of some drugs.
- State that digitalis is a heart drug that originates from foxgloves.
- State that aspirin is a painkiller that originates from willow.
- State that penicillin was discovered by Alexander Fleming and originates from the Penicillium mould.
- Recognise that many drugs were originally extracted from plants or microorganisms rather than being made synthetically from the start.
- Describe the sequence from discovery of a natural substance to development and testing before it becomes a medicine.
- State that many medicines originate from chemicals found in plants, giving a named example such as willow bark or foxgloves.
- Describe how a chemical is extracted from plant material and tested for a useful biological effect.
- Explain that chemists in the pharmaceutical industry synthesise the drug, often modifying the original molecule.
- Compare the plant-derived starting point with industrial synthesis, noting consistency, scale and purity.
- Recognise that synthesis allows structural modification to improve efficacy or reduce side effects.
- State that new drugs are tested and trialled before use to check safety and effectiveness.
- Describe preclinical testing using cells, tissues and live animals to assess toxicity and activity.
- Describe clinical trials involving healthy volunteers and patients, often comparing the drug with a placebo or existing treatment.
- Explain why trials are controlled, randomised or blinded to produce valid and reliable results.
- Link the purpose of testing to protecting patients from harm and confirming that the drug works.
- Define toxicity as the capacity of the drug to cause harm and state that it is tested at different concentrations.
- Define efficacy as the ability of the drug to produce the intended therapeutic effect.
- Explain that dose is tested to find an amount that is effective while minimising harmful side effects.
- Describe how a range of doses or concentrations is used to identify a suitable dose.
- Link the three tests together, noting that a useful drug must be effective at a dose that is not unacceptably toxic.
- State that preclinical testing occurs in a laboratory before testing in humans.
- Describe the use of cells, such as cultured cells, to test for effects on cell function or toxicity.
- Describe the use of tissues, such as isolated organ tissue, to test effects on a specific tissue response.
- Describe the use of live animals to study effects in a whole organism, including how the drug is absorbed or metabolised.
- Explain that preclinical results inform the decision about whether to proceed to clinical trials and raise ethical considerations.
- Healthy volunteers are used in early clinical trials to assess safety, tolerability and how the drug moves through the body.
- Patients with the target condition are used in later trials to test whether the drug treats that condition effectively.
- Using healthy volunteers first reduces the risk of a patient's illness masking or worsening drug side effects.
- Patients allow researchers to compare treated and untreated groups and judge whether the drug produces a real clinical benefit.
- Both groups are needed because safety data from healthy people and effectiveness data from patients answer different questions.
- Clinical trials in humans begin with very low doses to minimise the risk of serious harm because the drug's effects in humans are not yet known.
- Participants are monitored for safety, side effects and any therapeutic effect at each dose level.
- If the drug is found to be safe at low doses, further clinical trials are carried out with larger groups and/or different doses.
- Later trials aim to find the optimum dose, which is the dose giving the best balance of therapeutic benefit and acceptable side effects.
- The optimum dose is identified by comparing results across dose groups rather than by assuming that a higher dose is always better.
- Ethical and regulatory approval is required before each stage, and participants give informed consent.
Examiner Tips
- 💡Present the stages in a clear sequence: discovery, preclinical, clinical phases, licensing.
- 💡State the purpose of each stage, such as safety, dosage or effectiveness, rather than just naming it.
- 💡Mention placebos and randomisation when explaining how bias is reduced in clinical trials.
- 💡Name specific plant and microorganism sources to support your answer.
- 💡Use the terms extraction and active compound accurately.
- 💡Link traditional sources to modern drug development rather than treating them as separate topics.
- 💡Learn each drug with its source as a paired fact, for example digitalis and foxgloves, aspirin and willow, penicillin and Penicillium.
- 💡When a question asks for the scientist, give the full name Alexander Fleming rather than just a surname.
- 💡If asked why natural sources are important, link your answer to the discovery of new medicines and the need for testing.
- 💡Use the phrase starting point may still be a chemical extracted from a plant to show the link between natural sources and synthetic production.
- 💡Give one named plant example and state clearly what chemical or effect it provided.
- 💡When asked why chemists synthesise drugs, link your answer to scale, purity or modification rather than just saying it is easier.
- 💡Define safety and efficacy in your answer rather than repeating the words without explanation.
- 💡Sequence your answer: preclinical testing first, then clinical trials, then licensing or approval.
- 💡Mention at least one feature of fair testing, such as a control group, placebo or blinding, and say what it controls for.
- 💡Use the three headings toxicity, efficacy and dose to structure your answer and ensure all are covered.
- 💡Give a concrete example of efficacy, such as a drug lowering blood glucose in diabetes.
- 💡When discussing dose, mention both benefit and risk rather than only the beneficial effect.
- 💡Name all three systems, cells, tissues and live animals, because the statement specifically lists them.
- 💡Give one advantage of each system, for example cells allow many replicates while whole animals show whole-body effects.
- 💡If asked about ethics, state that animal testing is regulated and must be justified, without making unsupported claims.
- 💡Link each group to its purpose: healthy volunteers for safety, patients for effectiveness.
- 💡Use the phrase 'target condition' when explaining why patients are chosen.
- 💡If asked to compare, give one clear reason for each group rather than listing participants only.
- 💡When explaining why low doses are used first, link your answer to safety and the unknown effects of the drug in humans.
- 💡If asked how the optimum dose is found, describe comparing therapeutic effects and side effects across different dose groups.
- 💡Use the phrase 'optimum dose' precisely, and avoid writing that it is simply the highest dose tested.
Common Mistakes
- Reversing preclinical and clinical testing: correct this by stating that preclinical testing occurs before any human testing.
- Thinking clinical trials use only healthy volunteers: correct this by explaining that healthy volunteers are used in early safety trials, while patients are used to test effectiveness.
- Believing a drug is tested on humans as soon as it is discovered: correct this by describing the prior preclinical safety and toxicity checks.
- Thinking all drugs are synthetic: correct this by stating that many drugs were originally extracted from plants and microorganisms.
- Confusing the source organism with the drug itself: correct this by naming the organism and the drug separately, such as foxgloves and digitalis.
- Assuming traditional use proves effectiveness: correct this by explaining that modern testing is still needed to establish safety and efficacy.
- Confusing the source of aspirin with the source of digitalis; correct this by linking aspirin to willow and digitalis to foxgloves.
- Attributing the discovery of penicillin to the wrong scientist; correct this by naming Alexander Fleming and the Penicillium mould.
- Assuming a drug can be used as soon as it is discovered; correct this by stating that it must be tested for safety and effectiveness first.
- Assuming all drugs are completely artificial with no natural origin; correct this by stating that the starting point may be a plant chemical even when the final drug is synthesised.
- Confusing extraction with synthesis; extraction obtains a chemical from plant material, whereas synthesis builds the drug by chemical reactions.
- Believing that a plant chemical is automatically safe or effective; correct this by noting that it must still be tested and may be modified.
- Thinking that a drug only needs to be tested for safety; correct this by stating that efficacy must also be checked.
- Confusing preclinical testing with clinical trials; preclinical work uses cells, tissues and animals, while clinical trials involve human volunteers and patients.
- Assuming that a successful laboratory result means the drug is ready for patients; correct this by explaining that clinical trials and licensing are still required.
- Treating toxicity, efficacy and dose as the same thing; correct this by defining each separately and giving an example of what each test measures.
- Assuming that a higher dose always means a better effect; correct this by explaining that higher doses can increase toxicity and side effects.
- Thinking efficacy means safety; efficacy means the drug works, while safety relates to toxicity and side effects.
- Confusing preclinical testing with clinical trials; preclinical testing uses cells, tissues and animals, while clinical trials use humans.
- Thinking that animal testing is the only preclinical method; correct this by including cells and tissues as well.
- Ignoring the ethical dimension of animal testing; correct this by noting that it is regulated and used only when justified.
- Thinking clinical trials use only patients; correct this by stating that healthy volunteers are used first for safety testing.
- Confusing clinical trials with laboratory or animal testing; correct this by placing clinical trials after preclinical testing and involving human participants.
- Assuming healthy volunteers prove the drug works; correct this by explaining that effectiveness is tested in patients with the condition.
- Thinking that clinical trials start with the optimum dose: the error is skipping the low-dose safety stage; the correction is that very low doses are given first to check safety before doses are increased.
- Confusing the optimum dose with the maximum dose: the error is assuming the largest dose is best; the correction is that the optimum dose balances therapeutic effect against side effects.
- Believing that animal testing alone proves a drug is safe for humans: the error is ignoring species differences; the correction is that clinical trials in humans are still needed to assess safety and find the optimum dose.