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All drugs I remember tasting (with the notable exception of Aspirin) have bitter taste. Is the taste due to the active substance, or is a bittering agent added to them, perhaps to prevent overdose?

Take Paracetamol for instance: adult pills taste bitter, while the syrup for babies is sweet without any noticeable bitterness. Or is the dosage simply so low that I couldn't distinguish the drug taste behind all that sugar they put in?

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    $\begingroup$ Why do you think aspirin doesn't taste bitter? $\endgroup$ – jamesqf Dec 17 '20 at 17:02
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    $\begingroup$ Not sure why Dmitry said it, but I know that some Ibuprofen (who many call "aspirin") are covered in a sort of 'candy shell' which is bit sweet to my senses as well. I've never had Acetaminophen that were coated, but I use the Ibuprofen that is coated specifically because I don't have to rush the chaser to get the taste out so fast. $\endgroup$ – coblr Dec 17 '20 at 22:43
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    $\begingroup$ @jamesqf Because I tasted it. Aspirin is an acid and has a typical sour taste. $\endgroup$ – Dmitry Grigoryev Dec 18 '20 at 8:24
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    $\begingroup$ I’m voting to close this question because it is not about a problem in biology, but is solely about commercial/social marketing practice. It may possibly be relevant on SE Medical Sciences. $\endgroup$ – David Dec 18 '20 at 18:18
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    $\begingroup$ @Dmitry Grigoryev: Apparently your taste buds are more sophisticated, or perhaps I should say capable of greater discrimination, than mine are, because I certainly can't tell the difference. $\endgroup$ – jamesqf Dec 19 '20 at 2:21
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Short answer
A bittering agent may be applied to therapeuticals to prevent pediatric poisonings, but many drugs inherently taste bitter by themselves.

Background
Bitter taste is thought to have evolved as a way to decrease the risk of ingesting toxic substances, which may explain why many drugs taste bitter. In other words, classes of compounds that may harm the body often taste bitter (Menella et al., 2014). These include the often dangerous compounds like alkaloids and glycosides (source: University of Delhi). These include the familiar psychotropic compounds found in hallucinogenic plants, and the potentially deadly heart glycosides found in Digitalis species, respectively, among other classes of substances.

Paracetamol can be regarded as an alkaloid (Fig. 1), but not aspirin, as it lacks a nitrogen atom (Fig. 2). An alkaloid is defined as

any of a class of naturally occurring organic nitrogen-containing bases. Alkaloids have diverse and important physiological effects on humans and other animals. Well-known alkaloids include morphine, strychnine, quinine, ephedrine, and nicotine...

All of which are quite toxic. As cleverly noted in the comments, aspirin is not an alkaloid in its strictest sense, as it is not naturally occurring. However, it is chemically obtained by acetylation of salicylic acid (Mund et al., 2016), as shown in Fig. 2. Both of the reactants in themselves are naturally occurring; salicylic acid can be sourced from willow bark, and acetylation is a widely occurring biochemical reaction (Gibbs, 2017).

I myself have never had the chance to taste aspirin, unfortunately (although I performed the acetylation shown in Fig. 2 for my Organic Chemistry class :-). Originally, aspirin was developed from salicylic acid (Fig. 3) just because the taste of salicylic acid was so bitter. Acetylation diminished the bitterness, yet the anti-inflammatory action remained (Wu, 2000)). Besides reducing bitterness by chemical modification, masking the bitterness of drugs with, e.g.., sweeteners, can help to overcome the aversion to the bitter taste. This can aid in helping patients, especially pediatric ones, to take their medicine and adhere to their treatment regime (Chauhan, 2017).

Reversely, coming back to your question, adding bitter-tasting additives to drugs indeed seem to be an effective way to reduce pediatric poisonings (Menella et al., 2014)

References
- Chauhan, J stem cell Bio transplant (2014); 1(2): 12
- Gibbs, Trends Plant Sci (2015); 20(10): 599–601
- Menella et al., Clin Ther. 2013 Aug; 35(8): 1225–1246
- Mund et al., J Occup Med Toxicol (2016); 11: 32
- Wu, Circulation (2000); 102: 2022–3

paracetamol
Fig. 1. Paracetamol, or acetaminophen. source: Wikimedia Commons

Synthesis of Aspirin
Fig. 2. Synthesis of Aspirin, or acetylsalycilic acid, from salicylic acid. source: Imperial College, UK

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    $\begingroup$ I recall an advert in the 90s advertising a form of aspirin "without the aspirin taste" - it showed a bunch of people taking aspirin dissolved in water, screwing up their faces. I also recall experiencing the same. I can't find the advert anywhere however, and I haven't taken aspirin in years. $\endgroup$ – Phil Dec 17 '20 at 22:09
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    $\begingroup$ Interesting read: The importance of taste in pharmaceutical development but that yummy chewable baby aspirin now seems somehow evil... $\endgroup$ – uhoh Dec 18 '20 at 2:03
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    $\begingroup$ I like the answer except for the alkaloid part. Paracetamol does not appear to be naturally occurring. Aspartame seems to fit into the same (too broad) criteria, but is certainly not bitter. I think that taste is more complicated than it is made out to be here. $\endgroup$ – TAR86 Dec 18 '20 at 5:30
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    $\begingroup$ @RossPresser Notably, nowadays aspirin is the last thing you want to give to a kid because it might cause brain damage. $\endgroup$ – Dmitry Grigoryev Dec 18 '20 at 15:46
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    $\begingroup$ @TAR86 - thanks for your critical feedback. I have edited the answer - salicylic acid, the precursor for aspirin is extremely bitter, and also doesn't contain nitrogen - I have added this, and some history in my answer as well. Hope this helps. Thanks again! $\endgroup$ – AliceD Dec 18 '20 at 19:54

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