| Document and Entity Information | Jan. 31, 2023 |
| Cover [Abstract] | |
| Entity Registrant Name | SPECTRUM PHARMACEUTICALS INC |
| Amendment Flag | false |
| Entity Central Index Key | 0000831547 |
| Document Type | 8-K |
| Document Period End Date | Jan. 31, 2023 |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-35006 |
| Entity Tax Identification Number | 93-0979187 |
| Entity Address, Address Line One | Pilot House-Lewis Wharf, 2 Atlantic Avenue |
| Entity Address, Address Line Two | 6th Floor |
| Entity Address, City or Town | Boston |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02110 |
| City Area Code | (617) |
| Local Phone Number | 586-3900 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, $0.001 par value |
| Trading Symbol | SPPI |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
