Eichengrün has made his name through numerous inventions, such as processes for synthesizing chemical compounds. Aside from Aspirin, Eichengrün held 47 patents.
Bayer's official story credits Felix Hoffmann, a young Bayer chemist, with the invention of aspirin in 1897. Impure acetylsalicylic acid (ASA, the active compound of aspirin) had been synthesized already in 1853 by French chemist Charles Frédéric Gerhardt; the 1897 process developed at Bayer was the first to produce pure ASA that could be used for medical purposes.
Due to the rise of the Nazis in Germany, Eichengrün was unable to object when Hoffmann first made the claim that he (Hoffman) invented aspirin, in the footnote of a 1934 German Encyclopedia. Hoffmann's claim was once widely accepted, but many historians now consider it to be discredited.  Eichengrün first claimed to have invented aspirin in a 1944 letter from Theresienstadt concentration camp, addressed to IG Farben (of which Bayer was a part), where he cited his many contributions to the company (which was highly influential in the concentration camps), including the invention of aspirin, as reasons for why he should be released.
Five years later, Arthur Eichengrün published a paper in Pharmazie in 1949, where he explained that he had instructed Hoffmann to synthesise acetylsalicylic acid and that the latter had done so without knowing the purpose of the work. The paper elucidated how he planned and directed the synthesis of aspirin along with the synthesis of several related compounds, describing these events in detail. He also claimed to be responsible for aspirins initial surreptitious clinical testing. Finally, he explained that Hoffmanns role was restricted to the initial lab synthesis using his (Eichengrün's) process and nothing more.
Eichengrüns account was largely ignored by historians and chemists until 1999, when Walter Sneader of the Department of Pharmaceutical Sciences at the University of Strathclyde in Glasgow re-examined the case and came to the conclusion that indeed Eichengrüns account was convincing and correct and that Eichengrün deserved credit for the invention of aspirin. Bayer denied this in a press release, asserting that the invention of aspirin was due to Hoffmann.
;Evidence supporting Eichengrün's claims to the invention
Walter Sneader based his claims that Eichengrün both invented the process for synthesizing aspirin and oversaw its clinical testing on old and newly released archived materials, including letters, patents, and lab work. He found that Hoffmann was not credited with inventing the process for synthesizing Aspirin in any documents prior to 1934, 37 years after its initial synthesis. Further he found reason to doubt the footnotes credibility, not just for being published during the "Aryanization" period of Nazi Germany, but for its inaccurate claims about the testing of salicylic acid derivates other than acetyl ester. The vague reference did not specify which derivatives were tested, but claimed they had been discovered earlier but had been synthesized for "other purposes". No indication was given of what the others were, but in 1899 Heinrich Dreser, head of the experimental pharmacology laboratory at Elberfeld, named them in a publication as propionyl, butyryl, valeryl, and benzoyl salicylic acids. He further alluded to these derivatives in 1907 and again in 1918. However, the assertion that these salicylic acid derivates had been synthesized for non-therapeutic reasons is demonstrably false. Hoffmanns colleague Otto Bonhoeffer (who also worked under Eichengrün) had been awarded a US and UK patent in 1900 for several of these compounds. The patents indicate that the derivatives were prepared for the exact purpose of finding a salicylic acid derivative with therapeutic value. Sneader concluded that because of this error the 1934 footnote is unreliable.