Based on the information provided, there is no specific evidence indicating how antacids alter tigecycline's absorption. However, we can provide some context based on the general pharmacological properties of these drugs.

Tigecycline is an antibiotic used to treat various bacterial infections. Its absorption is not significantly affected by food, and it is not recommended to be co-administered with polyvalent cation-containing products, such as antacids, due to the possibility of a decrease in tigecycline absorption [1].

Antacids, typically containing aluminum, magnesium, or calcium salts, can affect the absorption of certain medications by altering the gastrointestinal pH or binding to the drugs [2][3]. This can lead to decreased bioavailability of the affected drug [3].

In summary, while there is no specific information on how antacids alter tigecycline's absorption, it is generally advised to avoid co-administering tigecycline with antacids due to the potential for decreased absorption resulting from the interaction between the antacids' cations and tigecycline [1].

Sources:
[1] DrugPatentWatch.com. (n.d.). Tradename: TIGECYCLINE. Retrieved from <https://www.drugpatentwatch.com/p/tradename/TIGECYCLINE>
[2] Hardman, J. G., & Limbird, L. E. (Eds.). (2001). Goodman & Gilman's the pharmacological basis of therapeutics (10th ed.). McGraw-Hill.
[3] FDA. (2010). Tigecycline (marketed as Tygacil) Information. Retrieved from <https://www.fda.gov/drugs/postmarket-drug-safety-information-patients-and-providers/tigecycline-marketed-tygacil-information>