Why most websites report frequency of adverse reactions of drugs(In terms of common, rare, etc..)? Does this indicate causal relationship? Like common adverse reactions is established to be induced by the drug and not as an adverse event? Like here in the case of cetirizine or reporting the frequency serves another purpose?
https://www.drugs.com/sfx/cetirizine-side-effects.html#refs
No ideal drug exist and treating something you have a chance to cause another harm, from two evils it is selected the less one, specifical indication for single disease and each drug is accompanied by side effect
Knowing how frequent is an adverse reaction or in other wards how prevalent and what incidence it occurs with can help us to a great extent in research, shaping and defining public policies, financial and pharmacoeconomic studies and so on. It is very important. It also help in the usual day to day clinical decision making. So, if i know that a cross reaction to antibiotic B where the patient is allergic to antibiotic A is more likely with another antibiotic C I will be using antibiotic B rather than C.
CIOMS generally gives this classification :
- Very common ADR occurs in more than 1/10 patients who take the drug
- Common (frequent) > = 1/100 and < 1/10
- Uncommon (infrequent) >= 1/1000 and < 1/100
- Rare >= 1/10000 and < 1/1000
- Very rare < 1/10000
The frequency of ADR is often determined during the clinical trials, although clinical trials are more designed to assess efficiency than to detect side effects (as clinical trials do not include more than 3000, it is for example impossible to see very rare ADR).
During those clinical trials ADR are assessed with an imputability method, as it is done in real life with pharmacovigilance.
When a patient has a symptom, those data helps health professionals to see if it could be or not an ADR, and if so stop or not the drugs if needed (and ask an expertise).
It doesn't tell causal relationship. If the adverse event report is increasing for a certain drug post-marketing (For an ADR not detected during clinical trial stage) tells us that controlled studies are required.
From a practice standpoint, frequency helps when communicating the likelihood of experiencing ADR to patients. However, severity of ADR may be more important for making decisions about whether to keep a drug in the market.
Frequency of an ADR is most helpful when you know the background incidence of the event. For instance, in a placebo controlled trial, the incidence (frequency) of headache may be 10% in the placebo arm and 10% in the active arm. As such, it is reasonable to conclude that headache is not likely related to the drug and is more likely related to the underlying condition.
However, if you have headache frequency 10% in placebo arm and 30% in active arm, then it is reasonable to assume that headache in your patient is likely related to drug.
Unfortunately, pharmacovigilance is an imperfect science. At a very basic level, patient reporting of adverse events is very variable, physician recording of adverse events is very variable, we often do not have control arms (especially in the real world). Sifting signals from the noise requires lots of data, lots of expertise, lots of experience, and lots of surveillance.
By knowing the frequency of a particular side effect the physician can prescribe a drug with confidence and at the same time he can avoid to write a drug with frequent dangerous effects.
We come to know the pharmacodynamic profile of a particular agent.
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