| Cover | Nov. 12, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Nov. 12, 2021 |
| Entity File Number | 001-38899 |
| Entity Registrant Name | MILESTONE PHARMACEUTICALS INC. |
| Entity Central Index Key | 0001408443 |
| Entity Tax Identification Number | 00-0000000 |
| Entity Incorporation, State or Country Code | A8 |
| Entity Address, Address Line One | 1111 Dr. Frederik-Philips Boulevard |
| Entity Address, Address Line Two | Suite 420 |
| Entity Address, City or Town | Montréal |
| Entity Address, State or Province | QC |
| Entity Address, Country | CA |
| Entity Address, Postal Zip Code | H4M 2X6 |
| City Area Code | 514 |
| Local Phone Number | 336-0444 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Shares |
| Trading Symbol | MIST |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Elected Not To Use the Extended Transition Period | true |
