Basis of Presentation and Consolidation—The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the U.S., or GAAP, and include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification, or ASC, and Accounting Standards Updates, or ASU, of the Financial Accounting Standards Board, or FASB.

Use of Estimates—The preparation of the consolidated financial statements in conformity with GAAP requires the Company’s management to make estimates and assumptions that affect the reported amounts of assets and liabilities at the date of the financial statements and the reported amount of expenses during the reporting period. Actual results could differ from those estimates. Management considers many factors in selecting appropriate financial accounting policies in

developing the estimates and assumptions that are used in the preparation of the financial statements. Management must apply significant judgment in this process. Management’s estimation process often may yield a range of potentially reasonable estimates and management must select an amount that falls within that range of reasonable estimates.

Revenue recognition—In June 2022, AMX0035 received marketing authorization with conditions as ALBRIOZA by Health Canada for the treatment of ALS, and the Company launched ALBRIOZA in Canada in July 2022. In September 2022, AMX0035 received approval as RELYVRIO by the FDA for the treatment of ALS in adults, and the Company launched RELYVRIO in the U.S. in October 2022. In 2024, the Company voluntarily discontinued the marketing authorizations for RELYVRIO and ALBRIOZA (AMX0035) for ALS and removed the product from the market based on topline results from the global Phase 3 PHOENIX trial, which did not meet its prespecified primary and secondary endpoints. Amylyx wound down the Open Label Extension as planned. As a result, the Company does not expect to generate revenue from the sale of RELYVRIO and ALBRIOZA in future periods.

Comprehensive Loss—Comprehensive loss includes net loss, as well as other changes in stockholders’ equity that result from transactions and economic events other than those with stockholders. Comprehensive loss is composed of net loss and other comprehensive (loss) income. Other comprehensive income (loss) consists of unrealized gains and losses on marketable securities and foreign currency translation.

Cash and Cash Equivalents—The Company considers all highly liquid investments with an original maturity of three months or less at the date of purchase to be cash equivalents. Cash equivalents represent funds invested in readily available checking and money market funds.

Restricted Cash Equivalents— Restricted cash equivalents consist of cash serving as collateral for a letter of credit issued for the Company’s office spaces and collateral for a corporate credit card program.

Accounts receivable, net— The Company’s accounts receivable consists of amounts due from customers related to product sales and have standard payment terms. The Company analyzes accounts that are past due for collectability and provides reserves against accounts receivable for expected credit losses that may result from a customer’s inability to pay. Amounts determined to be uncollectible are written-off against the established reserve. The credit losses were not material in the periods presented.

Marketable Securities—Marketable securities are composed of U.S. treasury bills. The Company classifies all of its marketable securities as available-for-sale. Accordingly, these investments are recorded at fair value, which is determined based on quoted market prices. Unrealized gains and losses on available-for-sale securities are included as a separate component of other accumulated comprehensive loss. The cost of marketable securities is adjusted for amortization of premiums and accretion of discounts. Such amortization and accretion are included in interest income. Realized gains and losses are included in other expense, net. The Company evaluates marketable securities for other-than-temporary impairment at the balance sheet date. Declines in fair value, if any, determined to be other than temporary-than-temporary are also included in other income, net.

When assessing marketable securities for other-than-temporary declines in value, the Company considers such factors as, among other things, how significant the decline in value is as a percentage of the original cost, how long the market value of the investment has been less than its original cost, and the Company’s ability and intent to retain the investment for a period of time sufficient to allow for any anticipated recovery in fair value and market conditions in general. There were no impairment charges on marketable securities in the periods presented.

Concentrations of Credit Risk—Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents and marketable securities. The Company maintains its cash in financial institutions that management believes have high credit quality. The Company has not experienced any losses on such accounts, and does not believe it is exposed to any unusual credit risk beyond the normal credit risk associated with commercial banking relationships.

Fair Value Measurements—Assets and liabilities recorded at fair value on a recurring basis on the consolidated balance sheet are categorized based upon the level of judgment associated with the inputs used to measure fair values. Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and

minimize the use of unobservable inputs. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows:

To the extent that the valuation is based on models or inputs that are less observable or unobservable in the market, the determination of fair value requires more judgment. Accordingly, the degree of judgment exercised by the Company in determining fair value is greatest for instruments categorized in Level 3. A financial instrument’s level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement.

The Company’s financial instruments consist of cash, cash equivalents, restricted cash, marketable securities, accounts receivable, net, accounts payable and accrued expenses. The Company’s marketable securities are carried at fair value, determined according to Level 1 and Level 2 inputs to the fair value hierarchy described above. The remaining financial instruments are stated at their respective carrying amounts, which approximate fair value due to the short-term nature of these assets and liabilities.

Property and Equipment, net—Property and equipment are stated at cost, net of accumulated depreciation. Depreciation of property and equipment is calculated using the straight-line method over the estimated useful lives of the respective assets. Maintenance and repairs that do not improve or extend the life of the assets are expensed when incurred. Upon sale or retirement of assets, the cost and accumulated depreciation are removed from the consolidated balance sheets and any resulting gain or loss is reflected in the consolidated statements of operations in the period realized. The range of useful lives of property and equipment is as follows:

Impairment of Long-Lived Assets—The Company evaluates assets for potential impairment when events or changes in circumstances indicate the carrying value of the assets may not be recoverable. Recoverability is measured by comparing the book values of the assets to the expected future net undiscounted cash flows that the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured by the amount by which the book values of the assets exceed fair value.

Business Combinations and Asset Acquisitions—The Company evaluates acquisitions of assets and other similar transactions to assess whether or not the transaction should be accounted for as a business combination or asset acquisition by first applying a screen test to determine if substantially all of the fair value of the gross assets acquired is concentrated in a single identifiable asset or group of similar identifiable assets. If the screen test is met, the transaction is accounted for as an asset acquisition. If the screen test is not met, further determination is required as to whether or not the Company has acquired inputs and processes that have the ability to create outputs which would meet the requirements of a business. If determined to be an asset acquisition, the Company accounts for the transaction under ASC 805-50, which requires the acquiring entity in an asset acquisition to recognize assets acquired and liabilities assumed based on the cost to the acquiring entity on a relative fair value basis, which includes transaction costs in addition to consideration given. Goodwill is not recognized in an asset acquisition and any excess consideration transferred over the fair value of the net assets acquired is allocated to the identifiable assets based on relative fair values. In-process research and development, or IPR&D, projects with no alternative future use are recorded in R&D expense upon acquisition, and contingent consideration obligations incurred in connection with an asset acquisition are recorded when it is probable that they will occur and they can be reasonably estimated.

License and Collaboration Agreements —The Company analyzes license and collaboration arrangements pursuant to ASC Topic 808 - Collaborative Arrangement Guidance and Considerations, or ASC 808, to assess whether such arrangements, or transactions between arrangement participants, involve joint operating activities performed by parties that are both active participants in the activities and exposed to significant risks and rewards dependent on the commercial success of such activities.

Collaboration arrangements often require upfront payments and may include additional milestone, research and development cost sharing, royalty or profit share payments, contingent upon the occurrence of certain future events linked to the success of the asset in development and commercialization. Upfront payments associated with collaborative arrangements and subsequent payments made to the partner for the achievement of development milestones prior to regulatory approval are expensed to acquired IPR&D expense as incurred. Contingent consideration obligations are recorded when it is probable that they will occur and they can be reasonably estimated.

Patent-Related Costs—Patent-related costs incurred in connection with filing and prosecuting patent applications are expensed as incurred due to the uncertainty about the recovery of the expenditure. Amounts incurred are classified as selling, general and administrative expenses in the accompanying consolidated statements of operations.

Stock-Based Compensation Expense—Stock-based compensation is recognized in the consolidated statements of operations based on the fair value on the date of grant over the requisite service period, which is generally equal to the vesting period of the respective award. Forfeitures are accounted for as incurred. Generally, the Company issues stock awards with only service-based vesting conditions and records the expense for these awards using the straight-line method. For awards subject to performance conditions, the Company recognizes stock-based compensation expense over the requisite service period using an accelerated recognition method when it is probable that the performance condition will be achieved. The Company classifies stock-based compensation expense in the same manner in which the award recipient’s payroll costs are classified.

The fair value of each restricted common stock award is measured based on the fair value of the Company’s common stock on the grant date.

The fair value of each option grant is estimated on the date of grant using the Black-Scholes option pricing model, which requires inputs based on certain subjective assumptions, including the expected stock price volatility, the expected term of the award, the risk-free interest rate, and expected dividends. The Company estimates its expected stock price volatility based on the historical volatility of publicly traded peer companies. The expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain vanilla” options. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. There is no expected dividend yield since the Company has never paid cash dividends on common stock and does not expect to pay any cash dividends in the foreseeable future. The stock price of the Company is based on the closing price on the date of grant.

Contingencies—From time to time, the Company may have certain contingent liabilities that arise in the ordinary course of business activities. The Company accrues for loss contingencies when losses become probable and are reasonably estimable. If the reasonable estimate of the loss is a range and no amount within the range is a better estimate, the minimum amount of the range is recorded as a liability on the Company’s consolidated balance sheets. The Company does not accrue for contingent losses that, in its judgment, are considered to be reasonably possible, but not probable; however, it discloses the range of reasonably possible losses.

Leases—The Company leases its offices, and may from time to time, enter into other lease agreements in conducting its business. The Company determines if an arrangement includes a lease at the inception of the agreement. For each of the Company’s lease arrangements, the Company records a right-of-use asset representing the Company’s right to use an underlying asset for the lease term and a lease liability representing the Company’s obligation to make lease payments. Operating lease right-of-use assets and operating lease liabilities are recognized at the lease commencement date based on the

net present value of the remaining future minimum lease payments over the lease term. If the interest rate implicit in the Company’s leases is not readily determinable, in determining the weighted-average discount rate used to calculate the net present value of lease payments, the Company utilizes an estimate of its incremental borrowing rate based on market sources including interest rates for companies with similar credit quality for agreements of similar duration to discount the lease payments. Lease expense for the Company’s operating leases is recognized on a straight-line basis over the lease term and variable lease costs are expensed as incurred. The Company did not have financing leases as of December 31, 2025 and 2024.

The Company elected the practical expedient not to apply the recognition and measurement requirements to short-term leases, which is any lease with a term of one year or less as of the lease commencement date. The lease may require the Company to pay additional amounts for maintenance and other expenses, which are generally referred to as non-lease components. Non-lease components (e.g., common area maintenance) are paid separately from rent based on actual costs incurred and therefore are not included in the operating lease right-of-use assets and lease liabilities and are reflected as an expense in the period incurred. If a lease includes options to extend the lease term, the Company does not assume the option will be exercised in its initial lease term assessment unless there is reasonable certainty that the Company will renew based on an assessment of economic factors present as of the lease commencement date.

Income Taxes—The Company accounts for income taxes using the asset and liability approach. Deferred tax assets and liabilities represent future tax consequences of temporary differences between the financial statement carrying amounts and the tax basis of assets and liabilities and for loss carryforwards using enacted tax rates expected to be in effect in the years in which the differences reverse. A valuation allowance is established to reduce deferred tax assets to the amounts expected to be realized. The Company also recognizes a tax benefit from uncertain tax positions only if it is “more likely than not” that the position is sustainable based on its technical merits. The Company accounts for interest and penalties related to uncertain tax positions as part of its provision for income taxes. To date, the Company has not incurred material interest and penalties related to income tax positions.

Valuation allowances are provided, if, based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. As of December 31, 2025, we continued to maintain a full valuation allowance against all of our deferred tax assets based on management’s evaluation of all available evidence, including our history of incurring significant losses from operations.

New Accounting Pronouncements Not Yet Adopted

In November 2024, the FASB issued ASU No. 2024-03, Income Statement – Reporting Comprehensive Income – Expense Disaggregation Disclosures (Subtopic 220-40): Disaggregation of Income Statement Expenses, which requires new financial statement disclosures in tabular format, in the notes to financial statements, of specified information about certain costs and expenses. The amendments in this update do not change or remove current expense disclosure requirements. The amendments in this update are effective for the Company's annual financial statement disclosure beginning December 31, 2027, and interim periods within the years beginning January 1, 2028. Early adoption is permitted. The Company is currently evaluating the impact of this ASU on its consolidated financial statements and related disclosures.

Recently Adopted Accounting Pronouncements

In December 2023, the FASB issued ASU No. 2023-09, Income Taxes (Topic 740): Improvement to Income Tax Disclosures, or ASU 2023-09, to enhance the transparency and decision usefulness of income tax disclosures. ASU 2023-09 is effective for the Company beginning January 1, 2025 on a prospective basis. The Company adopted ASU 2023-09, which did not have a material impact on its consolidated financial statements and related disclosures.