Additive effect in pharmacology describes the situation when the combining effects of two drugs equal the sum of the effects of the two drugs acting independently. The concept of additive effect is derived from the concept of synergy. It was introduced by the scientists in pharmacology and biochemistry fields in the process of understanding the synergistic interaction between drugs and chemicals over the century.
Additive effect often occurs when two similar drugs are taken together to achieve the same degree of therapeutic effect while reducing the specific adverse effect of one particular drug. For example, aspirin, paracetamol, and caffeine are formulated together to treat pain caused by tension headaches and migraine.
Additive effect can be used to detect synergy as it can be considered as the baseline effect in methods determining whether drugs have synergistic effect. Synergistic effect is similar to additive effect, having a combination effect greater than additive effect. It can produce an effect of 2+2 > 4 when two drugs are used together. Additive effect can also be found in a majority of combination therapies, although synergistic effect is more common. If the combination of two drugs in combination therapy has an effect lower than the sum of the effects of the two drugs acting independently, also known as antagonistic effect, the drugs will seldom be prescribed together in the same therapy.
Drug or chemical combinations with additive effects can cause adverse effects. For example, co-administration of non-steroidal anti-inflammatory drugs (NSAIDs) and glucocorticoids increases the risk of gastric bleeding.