BMS 113 · Common Core (Tronc commun)Section 2.4 · Sources of drugs
Section 8 of 31
Synthetic and semi-synthetic drugs
Distinguish a semi-synthetic from a fully synthetic drug, name the main synthetic classes, and explain why most modern drugs come from synthetic sources.
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Why this matters
If you miss this, you may think natural and synthetic medicines are opposite kinds of product, when most modern drugs are semi-synthetic or synthetic. The vocabulary here lets you read a formulary, understand where supply comes from and judge claims about natural safety. It connects back to Unit 2.1 on plant molecules and forward to Unit 2.6 on drug names.
What you need to know first
You should already know that a molecule structure decides what it does in the body, and that plants, animals and microorganisms are natural sources of drugs. Unit 2.3 explained how a living cell can be instructed to make a substance.
Words to know (4)
Semi-synthetic drug
A drug made by chemically changing a molecule that already exists in nature. Amoxicillin is a semi-synthetic modification of Penicillin G.
Fully synthetic drug
A drug that does not exist in nature and is built entirely in the laboratory from simpler chemicals. A laboratory-made analgesic with no natural counterpart is fully synthetic.
Natural drug
A drug obtained from a plant, an animal, a mineral or a microorganism with little or no chemical change. An alkaloid extracted unchanged from a plant is a natural drug.
Therapeutic class
A group of medicines used for the same kind of problem, such as antibiotics or antihypertensives. Antimalarials are the therapeutic class that includes quinine and artemisinin.
One idea at a timeIdea 1 / 4
In one sentence A semi-synthetic drug is a natural molecule that chemists have changed into a new medicine with its own properties.
A semi-synthetic drug begins as a natural molecule
Most naturally occurring drugs can also be made or altered in the laboratory. A semi-synthetic drug is produced by chemical modification of a molecule that already exists in nature: the natural substance is the starting point, and chemistry changes part of it. Amoxicillin and ampicillin, for example, are chemical modifications of Penicillin G. The modification is not cosmetic; it produces a new medicine with its own properties, its own dose and its own name. Semi-synthetic drugs therefore sit between the natural and the fully synthetic worlds: nature supplies the scaffold and the laboratory improves it. Because the starting material usually comes from a fermentation, supply still depends on a living production system.
For example
Penicillin G is produced from a mould, and chemists then modify it chemically to obtain amoxicillin or ampicillin, which are different medicines with different properties.
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What makes amoxicillin semi-synthetic rather than natural?
Show the answer
It is produced by chemically modifying Penicillin G, a molecule that comes from nature.
Read detailed explanations
Why chemists modify a natural molecule
Modification is done for practical reasons. A natural molecule may be absorbed poorly, destroyed by stomach acid, eliminated quickly by the body, or inactive against organisms that have learned to resist it. Chemical modification can change each of these properties: adding, removing or replacing a chemical group alters how the molecule crosses membranes, how long it survives and how well it binds its target. Penicillin G, obtained from a mould, is the classic starting point, and amoxicillin and ampicillin are modifications that give the same family different behaviour. This is why semi-synthetic drugs are so numerous: instead of searching for a completely new substance, chemists can improve a known one. The consequence for practice is that drugs in the same family are not interchangeable. A modification that improves one property may also change the spectrum of activity or the risk of allergy, so each member of the family must be learned on its own terms rather than as “more of the same”.
Vocabulary: natural, semi-synthetic, synthetic
Three labels must not be confused. A natural drug is obtained from a plant, an animal, a mineral or a microorganism with little or no chemical change. A semi-synthetic drug is made by chemically modifying a molecule that comes from such a natural source. A fully synthetic drug is made entirely in the laboratory and does not exist in nature. The boundaries matter when you read a package insert or a formulary. A medicine described as semi-synthetic still depends on a natural raw material, so its supply may be affected by fermentation capacity, whereas a fully synthetic medicine depends only on chemical production and its reagents. The three labels say nothing by themselves about safety, cost or efficacy: they describe origin and production, and each origin brings its own quality-control problems — plant variability, animal contamination, culture control or chemical purity.
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Semi-synthetic and synthetic drugs
Penicillin G from the mould, chemically modified into semi-synthetic antibiotics, beside the fully synthetic classes that do not exist in nature.
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01 / 5Penicillin G, the natural starting point
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Penicillin G, the natural starting point
Penicillin G is the natural antibiotic produced by the mould Penicillium. It is obtained by fermentation rather than by chemical synthesis, and it remains the starting material from which several widely used antibiotics are prepared in the laboratory.
Schematic; not to scale. Classes overlap in practice — some antibiotics are semi-synthetic and some antiviral or antimalarial drugs are derived from natural molecules; the conclusion repeats the statement of the lecture notes.
Key points to remember
✓A semi-synthetic drug starts from a natural molecule and is changed by chemistry into a new medicine.
✓A fully synthetic drug does not exist in nature and is built entirely in the laboratory.
✓Synthetic and semi-synthetic drugs cover every major therapeutic class, from pain to malaria.
✓Most medicines in use today are synthetic or semi-synthetic, and origin says nothing about safety.
Go deeper
Worked example
Labelling a molecule by its origin. Four substances are placed on your bench: an alkaloid extracted unchanged from a plant, Penicillin G from a mould, amoxicillin made by modifying Penicillin G, and a molecule designed and built entirely in the laboratory. You must label each as natural, semi-synthetic or synthetic, and justify the label.
The plant alkaloid is natural: it is obtained from a living source with little or no chemical change, so its structure is exactly the one nature made.
Penicillin G is also a natural product of a microorganism, but it is the starting material for the next substance, which is why it matters here.
Amoxicillin is semi-synthetic: it is a chemical modification of Penicillin G, so a natural molecule is the starting point and chemistry changes part of it.
The designed molecule is fully synthetic: it does not exist in nature, so every part of it was made in the laboratory from simpler chemicals.
Check the therapeutic class of each before you finish: analgesics, antiretrovirals, antihypertensives, anti-asthmatics, antibiotics, anti-epileptics, antifungals, antiparasitics and antimalarials all contain synthetic or semi-synthetic members.
The label natural, semi-synthetic or synthetic describes where the molecule comes from and how it was made, not how safe or how strong it is. Amoxicillin and Penicillin G are related, but they are different medicines with different names and properties.
Common misconception
Many beginners assume that because a drug is synthetic it is “less natural” and therefore more dangerous than a plant remedy. Toxicity depends on the molecule and the dose, not on its origin: a natural alkaloid and a synthetic analgesic can each be safe at one dose and poisonous at another.
Check your reasoning
A patient asks whether a semi-synthetic antibiotic is “the same thing” as the natural penicillin it was made from. What do you answer?
Reveal an answer
No. The semi-synthetic drug began as the natural molecule, but its structure was chemically changed, so it is a different substance with its own properties, dose and adverse effects. Amoxicillin and ampicillin are modifications of Penicillin G, not identical copies of it, which is why they carry different names and are prescribed on their own terms.
Where this comes from
BMS 113 General Pharmacology I lecture notes · Unit 2, p. 13
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About this course and its sources
Bilingual lessons organised from the supplied BMS 113 General Pharmacology I lecture notes. Diagrams are original educational schematics; pharmacology, nursing and French-language review are pending.