Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2022	Aug. 12, 2022
Entity Addresses [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2022
Document Fiscal Year Focus	2022
Document Fiscal Period Focus	Q2
Trading Symbol	ALNA
Entity Registrant Name	ALLENA PHARMACEUTICALS, INC.
Entity Central Index Key	0001624658
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		122,083,226
Entity File Number	001-38268
Entity Tax Identification Number	45-2729920
Entity Address, Address Line One	142-F North Road
Entity Address, Address Line Two	Suite 150
Entity Address, City or Town	Sudbury
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	01776
City Area Code	617
Local Phone Number	467-4577
Title of 12(b) Security	Common Stock, par value $0.001 per share
Security Exchange Name	NASDAQ
Document Quarterly Report	true
Document Transition Report	false
Entity Incorporation, State or Country Code	DE
Former Address
Entity Addresses [Line Items]
Entity Address, Address Line One	One Newton Executive Park
Entity Address, Address Line Two	Suite 202
Entity Address, City or Town	Newton
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02462