The terms synergistic effect (synergism), antagonistic effect (antagonism) and additive effect are all clinical and pharmacological phrases that have clinical applications in drug administration in real life. They are also applied in research particularly in novel drug discovery, drug testing and therapeutic drug applications.
Chemical reactions in chemistry happen when two compounds or more react together to form a new compound known as the product of the reaction. Additive effect, antagonistic effect, and synergistic effect are all types of chemical reactions or pharmacological outcomes that can be expected when two or more drugs are combined in vivo or in vitro to achieve certain therapeutic outcome.
When pharmacological compounds including antibiotics or drugs are combined for therapeutic or experimental purposes, there is usually three types of pharmacological outcomes or effects that can be expected. These pharmacological outcomes are: (1) synergism or synergistic effects, (2) antagonism or antagonistic effects and (3) additive effects.
Synergism or synergistic effect
Synergism is the combination effect of a drug that is greater than the additive effect. It can also be called synergistic effect. Two drugs (e.g., drug A and drug B) exert or experience synergistic effect when the pharmacological effect of the two drugs or antibiotics (drugs A and B) in a combination is found to be greater than the pharmacological effect seen when the sum of each of the pharmacological outcome of each individual drug or compound (i.e., drugs A or drugs B) is calculated or summed up.
In this case, a double effect of both drug A and drug B is achieved. This means that when drug A is combined with drug B and both drugs (A and B) are co-administered in vivo or in vitro at same time, the pharmacological effect or outcome that will be experienced will be greater than the pharmacological effect recorded when each of the drugs are used individually and their pharmacological effects summed up.
Clinically and experimentally, when two drugs or antibiotics are administered, the therapeutic or pharmacological effects observed is greater and higher than the sum of the effects of each antibiotic when administered or taken individually. This is usually common in drugs or antibiotics from different classes. Clinically, there are two possible outcomes when drugs are combined to achieve synergism:
- Positive outcome
- Negative outcome
When the outcome is positive, there is usually an improved treatment outcome. But when the effects are negative, there is usually an unwanted clinical, pharmacological or experimental outcome of toxicity. Negative outcomes of synergism usually result in adverse or untoward effects of drugs while positive outcome of drugs usually results in enhanced drug efficacy.
Example of synergistic interaction – The effects of the combination will be greater than the sum of the effects of each chemical (or drug taken separately). The combination is usually more than double the effect. In this case, for example, 2 + 2 = 10.
Additive effect
Additive effect of a drug is a pharmacological outcome or effect that is similar to the effects of the individual compounds. For example, the effect of the two drugs/antibiotics equals the sum of the effects of the two drugs acting independently.
Additive effect is common in compounds/drugs/antibiotics that have similar chemical structures. For example, when penicillin interacts with ampicillin, a pharmacological effect known as additive effect can be observed. Both penicillin and ampicillin belong to the same antibiotic class and have a similar chemical structure known as beta-lactam ring.
This means that if a person takes both penicillin and ampicillin at the same time, the individual will get a pharmacological effect (of cell wall inhibition of the target pathogen within the body) or outcome that is similar to the effect of the individual compounds. And this will be the same effect when the person takes each drug (penicillin or ampicillin) separately or individually.
Example of additive interaction – The effects of the combination will equal the sum of the effects of each chemical (or drug taken separately). In this case, for example, 2 + 2 = 4.
Antagonistic effect
Antagonistic effect is a pharmacological combination effect of a drug combination that is less than the additive effect of a drug. It can also be called antagonism. In antagonism or antagonistic effect, the sum or combination effects of two drugs or antibiotics is less than the sum of the effects when each of the drugs is administered or taken individually or separately.
Clinically, this can be seen when one drug (in a combination) is expected to delay the excretion of the other drug (in the combination) in order to increase the therapeutic potency of the latter drug. Antagonism can also be seen clinically when a drug blocks the toxicity of another drug or when a drug increases the excretion of another drug in the combination.
In antagonism, the two drugs usually cancel each other out or help the other drug to perform its function better by either delaying or increasing excretion, retention or therapeutic action in the body.
Example of antagonistic Interaction – The effects of the combination will be less than the sum of the effects of each chemical (or drug taken separately). One drug can weaken the effects of another drug or even cancel out the other drug’s effect to produce a net effect of zero. In this case, for example, 2 + (- 2) = 0.
