Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2026	May 07, 2026
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Mar. 31, 2026
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2026
Entity File Number	001-42831
Entity Registrant Name	LB Pharmaceuticals Inc
Document Fiscal Period Focus	Q1
Entity Central Index Key	0001691082
Entity Tax Identification Number	81-1854347
Entity Incorporation, State or Country Code	DE
Entity Address, Address Line One	One Pennsylvania Plaza
Entity Address, Address Line Two	Suite 1025
Entity Address, City or Town	New York
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10119
City Area Code	212
Local Phone Number	605-0300
Title of 12(b) Security	Common Stock, $0.0001 par value
Trading Symbol	LBRX
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		28,676,652