Forward-Looking Statements and Non-GAAP Financial Information Certain statements set forth in this presentation constitute "forward-looking statements" within the meaning of the Private Securities Litigation Reform Act o

March 25, 2021 Alkermes Investor Day Exhibit 99.1

Forward-Looking Statements and Non-GAAP Financial Information Certain statements set forth in this presentation constitute "forward-looking statements" within the meaning of the Private Securities Litigation Reform Act of 1995, as amended, including, but not limited to, statements concerning: the company's expectations with respect to its current and future financial and operating performance, business plans or prospects, including revenue growth from its current commercial product portfolio and the addition of potential new revenue streams, and the company's plans and ability to manage for growth and profitability, including achievement of its stated profitability targets, through revenue growth, expense management and optimization of its cost structure and exploration of strategic opportunities; the potential therapeutic and commercial value of the company's marketed products and development candidates; expectations regarding patient life for the company's products; expectations regarding the effectiveness and cost-efficiency of the company's research and development ("R&D") strategy and the potential of the company's R&D capabilities, including its molecule design and engineering capabilities; timelines, plans and expectations for development activities relating to the company's products and development candidates, including (i) for nemvaleukin alfa ("nemvaleukin"), plans to initiate studies in mucosal melanoma and platinum-resistant ovarian cancer to support potential registration, further explore the potential synergistic benefits of nemvaleukin to existing cancer treatments, and pursue strategic collaborations, (ii) for ALKS 1140, plans to initiate phase 1 first-in-human trials, (iii) and for the company's other early-stage programs, including its orexin program, its engineered cytokine platform, including IL-12 and IL-18, and its HDAC inhibitor platform, plans to conduct proof of concept studies and/or other enabling activities, advance towards characterization and nomination of lead candidates, and identify and prioritize initial potential indications; the company's expectations relating to regulatory activities and interactions, including the U.S. Food and Drug Administration's ("FDA") PDUFA target action date for the company's new drug application ("NDA") for LYBALVI and plans to advance discussions on registration plans for nemvaleukin; expectations concerning commercial activities relating to the company's products and product candidates, including preparations for the potential commercial launch of LYBALVI. The company cautions that forward-looking statements are inherently uncertain. These risks, assumptions and uncertainties include, among others: the impacts of the ongoing COVID-19 pandemic and continued efforts to mitigate its spread on the company's business, results of operations or financial condition; the unfavorable outcome of litigation, including so-called "Paragraph IV" litigation and other patent litigation, related to any of the company's products, which may lead to competition from generic drug manufacturers; the FDA may not agree with the company's regulatory approval strategies or components of the company's NDAs, including clinical trial designs, conduct and methodologies, manufacturing processes and facilities, or the adequacy of the data or other information included in the company's regulatory submissions to support the FDA's requirements for approval, and may make adverse decisions regarding the company's products, including with respect to the NDA for LYBALVI; the company's development activities may not be completed on time or at all; the results of the company's development activities may not be positive or predictive of real-world results, and preliminary data from ongoing studies may not be predictive of future or final data from such studies, results of future studies or real-world results; the company and its licensees may not be able to successfully commercialize their products or support growth of revenue from such products; there may be a reduction in payment rate or reimbursement for the company's products or an increase in the company's financial obligations to governmental payers; the company's products may prove difficult to manufacture, be precluded from commercialization by the proprietary rights of third parties, or have unintended side effects, adverse reactions or incidents of misuse; and those risks, assumptions and uncertainties described under the heading "Risk Factors" in the company's Annual Report on Form 10-K for the year ended Dec. 31, 2020 and in subsequent filings made by the company with the U.S. Securities and Exchange Commission ("SEC"), which are available on the SEC's website at www.sec.gov and on the company's website at www.alkermes.com in the "Investors-SEC filings" section. Existing and prospective investors are cautioned not to place undue reliance on these forward-looking statements, which speak only as of the date hereof. Except as required by law, the company disclaims any intention or responsibility for updating or revising any forward-looking statements contained in this presentation.

Non-GAAP Financial Measures: This presentation includes information about certain financial measures that are not prepared in accordance with generally accepted accounting principles in the U.S. (GAAP), including non-GAAP net income and EBITDA. These non-GAAP measures are not based on any standardized methodology prescribed by GAAP and are not necessarily comparable to similar measures presented by other companies. Reconciliations of non-GAAP financial measures to the most directly comparable GAAP financial measures, to the extent reasonably determinable, can be found in the Appendix of this presentation.

Note Regarding Trademarks: The company and its affiliates are the owners of various U.S. federal trademark registrations ( ) and other trademarks (TM), including ARISTADA , ARISTADA INITIO , VIVITROL , and LYBALVITM. VUMERITY is a registered trademark of Biogen MA Inc., used by Alkermes under license. Any other trademarks referred to in this presentation are the property of their respective owners. Appearances of such other trademarks herein should not be construed as any indicator that their respective owners will not assert their rights thereto.

Richard Pops Chief Executive Officer Scientific and Business Excellence: Focused on New Approaches to Creating Value

Diversified Biopharmaceutical Company With Proven Drug Development and Commercialization Capabilities Significant, diverse revenues driving >$1B topline and positioned for growth

Proprietary commercial products that target large markets in addiction and psychiatry Additional potential revenue streams as new products launch* and grow Licensed Product (royalty & manufacturing revenue) Pipeline of novel development candidates designed to target significant unmet needs ALKS 1140

Neurodegenerative and neurodevelopmental disorders

Narcolepsy Nemvaleukin alfa

Advanced solid tumors

Advanced solid tumors Focus on Profitability Oncology Neuroscience Focus on driving cost efficiencies and operating leverage while investing in the long-term growth of high-potential commercial and development-stage products *LYBALVI NDA under FDA review; PDUFA June 1, 2021

Target evaluation driven by potential to add new medical value to established medications

Enabled rapid advancement to late-stage development

Yielded VIVITROL , ARISTADA , VUMERITY , LYBALVI *

Evolution of Alkermes R&D Strategy Advanced formulation science, prodrug chemistry and drug delivery technologies applied to improve established pharmacology *LYBALVI NDA under FDA review; PDUFA June 1, 2021

Molecular design, medicinal chemistry and protein engineering expertise applied to develop novel molecules to address unmet needs in neuroscience and oncology Scientific Excellence New R&D Strategy

Two areas of scientific expertise: Neuroscience and Oncology

Programs with validated biology rationale

Leverage established small molecule and protein design capabilities

Utilize an integrated approach to target evaluation

Design development programs to provide key data early

Prioritize and allocate resources based on data Evolution of Alkermes R&D Strategy: Focus & Management New Molecules Leveraging Scientific Capabilities

Business Excellence Value Enhancement Plan: Growth, Efficiency and Profitability Value Enhancement Plan

Establishes clear profitability targets

Provides framework for capital allocation decisions and prioritization of investments

Focuses investment on highest-ROI opportunities emerging from new R&D platform

Incorporates feedback from shareholders, external advisors and peer-group benchmarking data Management and governance focused on growth, efficiency, and profitability

Innovation-driven value creation and profitability Scientific Excellence Business Excellence Scientific & Business Excellence Drive Enterprise Value

Consistent With Alkermes' Purpose and Values

Today's Agenda VALUE CREATION Innovation, Growth and Profitability - Blair Jackson, Executive Vice President and Chief Operating Officer

ALKERMES' NEW R&D MODEL Optimizing for Success - Craig Hopkinson, M.D., Executive Vice President and Chief Medical Officer Focus on Innovative Molecular Design - Markus Haeberlein, Ph.D., Senior Vice President, Research

ONCOLOGY Nemvaleukin alfa: Clinical Data Updates - Jessicca Rege, Ph.D., Vice President, Clinical Research, Oncology Nemvaleukin alfa: Preclinical Research - Heather Losey, Ph.D., Senior Director, Research, Oncology Tumor-Targeted Split IL-12 Fusion Protein - Josh Heiber, Ph.D., Principal Scientist, Research, Oncology

NEUROSCIENCE Selective HDAC Inhibitors - Markus Haeberlein, Ph.D., Senior Vice President, Research Orexin 2 Receptor Agonists - Brian Raymer, Ph.D., Senior Director, Research Project Leadership and Strategy

Blair Jackson Chief Operating Officer Value Creation at Alkermes: Innovation, Growth and Profitability

Strong Operational Foundation to Drive Value Creation Revenues: Diverse and positioned for growth Commitment to Profitability: Focused on organizational efficiency and cost management

Focused on highest-ROI opportunities Strong Corporate Governance: Board refreshment, independence and oversight *2021 reflects the midpoint of our NGNI expectations. These expectations, and the underlying assumptions and risks, are set forth in Alkermes plc's (the "Company") Current Report on Form 8-K ("8-K") filed with the SEC on February 11, 2021. The Company expressly disclaims any obligation to update or reaffirm these expectations.

**Reconciliations of non-GAAP financial measures to the most directly comparable GAAP financial measures, to the extent reasonably determinable, can be found in the Appendix of this presentation.

NGNI: Non-GAAP net income Illustrative only.

Revenues: Growing Commercial Products Serving Large Markets

Topline Growth and Diversification Reflect Evolving Business * Inclusive of ARISTADA INITIO .

**VUMERITY developed by Alkermes and licensed to Biogen.

Current proprietary commercial products Proprietary candidate under FDA review; PDUFA June 1 Licensed Product (royalty & manufacturing revenue) Expected Growth Drivers Patent Life 2020 Net sales CAGR

Indications Alcohol dependence (AD)

Prevention of relapse to opioid dependence (OD) following opioid detoxification

Schizophrenia Schizophrenia

Bipolar I disorder Relapsing forms of multiple sclerosis (MS) $311M

2016-2020 Potential launch in 2021 $23M

Launched in Q4'19 2029* 2035 2031 2033 *Under the terms of a settlement and license agreement entered into in July 2019 with Amneal Pharmaceuticals LLC ("Amneal"), Alkermes granted Amneal a non-exclusive license under certain patents covering VIVITROL, including the remaining patent covering VIVITROL in the U.S., to market and sell a generic formulation of VIVITROL in the U.S. beginning sometime in 2028 or earlier under certain circumstances.

Subject to Paragraph IV certification related to an ANDA seeking FDA approval of a generic version. A patent infringement lawsuit has been filed against the Paragraph IV filer in response. Full indication and prescribing information for VIVITROL may be found at www.vivitrol.com Full prescribing information for ARISTADA may be found at www.aristada.com (Commercialized by Biogen)

VIVITROL : Growth in Opioid Dependence and Alcohol Dependence 23%

2015-2019 COVID-19 Impact +54% +38% +31% +6% +45% VIVITROL Net Sales ($M) VIVITROL Growth in Alcohol Dependence COVID-19 interrupted 5 consecutive years of strong brand growth Alcohol dependence indication - strong source of recent growth

VIVITROL : Anticipated Recovery Following COVID-19 Disruption Alcohol dependence*: Resumed growth as new patient starts rebounded from pandemic lows in Q2'20

Opioid dependence*: Volumes have stabilized but continue to be impacted by COVID-19-related disruptions to the treatment landscape

Reduced patient capacity and treatment services at settings of care where people commonly undergo opioid detoxification (e.g. residential treatment centers, correctional facilities)

VIVITROL volume expected to regain growth as access to care improves Monthly VIVITROL Units VIVITROL stabilization and recovery began COVID-19 disruptions began *Full indication and prescribing information for VIVITROL may be found at www.vivitrol.com

CAGR ARISTADA : Strong Growth Driven by Two-Month Dose * Inclusive of ARISTADA INITIO ; TRx Data: IQVIA NPA data Dec R3; MOT: Months of therapy ARISTADA Net Sales ($M) ARISTADA 1064 mg TRx MOT increased ~50% in 2020* Strong performance reflects favorable product characteristics Growth of 2-month 1064 mg dose reflects differentiated value proposition

aLAIs Branded Market Size* Annual TRx** Market Share** ~$3.5B ~2M 9% ARISTADA MOT Share ARISTADA TRx MOT Growth Rate ARISTADA : Growth Outpaced Atypical Long-Acting Injectable (aLAI) Antipsychotic Market *Includes ARISTADA , Abilify Maintena (estimated sales), Invega Sustenna/Trinza , Risperdal Consta and Perseris .

MOT: Months of therapy

Alkermes Value Proposition

Drive operating leverage from established psychiatry commercial infrastructure

Expected to be accretive in year 2 of launch

*The brand name LYBALVI has been conditionally accepted by the FDA and will be confirmed upon approval.

** NDA resubmission under review following FDA Complete Response Letter and records requests relating to manufacturing of LYBALVI.

Launch Planning Target well-defined healthcare provider call universe at launch

Implement patient access programs designed to mitigate payer restrictions early in launch LYBALVI *: Potential New Revenue Driver in Oral Atypical Antipsychotic Market Daily oral investigational antipsychotic designed to offer efficacy of olanzapine for adults with schizophrenia and adults with bipolar I disorder

Addition of samidorphan intended to mitigate olanzapine-associated weight gain

PDUFA date June 1, 2021**

Anticipated launch H2 2021

ENLIGHTEN-2: Multicenter, double-blind, randomized, phase 3 study that evaluated the weight gain profile of LYBALVI (ALKS 3831) compared to olanzapine over six months in 561 patients with stable schizophrenia.

Most common adverse events for LYBALVI: weight gain, somnolence & dry mouth. Most common adverse events for olanzapine: weight gain, somnolence & increased appetite. higher mean percent weight change at six months for patients who received olanzapine vs. LYBALVI

57% ENLIGHTEN-2 Phase 3 Results *LYBALVI NDA under FDA review; PDUFA June 1, 2021 the risk of clinically meaningful weight gain ( 10%) from baseline (29.8% for olanzapine vs. 17.8% for LYBALVI) 2.0x

ORAL AAPs BIPOLAR I DISORDER Branded Market Size1 Annual TRx2 Market Share4 $1.5B ~15M 21% olanzapine $1.7B ~15M 12% olanzapine ORAL AAPs SCHIZOPHRENIA* Commercial Payer %3 20% 39% ~70K treatment switches occur each month5 olanzapine NBRx growth year-over-year6

15% Oral Atypical Antipsychotic (AAP) Markets Represent Large Opportunities 1IQVIA reported sales (NSP Audit Dec 2020) factored by indication using IQVIA reported indication mix (SOB Dec 2020); IQVIA reported sales are higher than manufacturer reported net sales because IQVIA does not incorporate all gross-to-net expenses 2 IQVIA reported TRxs (NPA Audit Dec 2020) factored by indication using IQVIA reported indication mix (SOB Dec 2020) 3 IQVIA SOB Data, Q4 2020 4 IQVIA reported TRxs (NPA Audit Dec 2020) converted into MOT and factored by indication using IQVIA reported indication mix (SOB Dec 2020) 5IQVIA SOB data for the oral atypical antipsychotic market, specific to SZ/BD indications. SZ data includes schizophrenia and schizoaffective disorder, BD data includes BD1, BD2 and other BD indications, switches includes add-on 6IQVIA SOB data Dec 2020, Dec'20 vs. Dec'19, all indications

*Due to data limitations, schizophrenia data includes schizoaffective disorder (for which LYBALVI will not be indicated) Due to data limitations, BD1 data includes other BD indications (for which LYBALVI will not be indicated)

Novel oral fumarate for the treatment of relapsing forms of multiple sclerosis (MS)

Biogen holds exclusive, worldwide license to commercialize

15% royalty to Alkermes on worldwide net sales

~$8B oral MS market* *IQVIA NPA TRx data. VUMERITY Growth Presents Additional Long-Term Revenue Opportunity

*These expectations, and the underlying assumptions and risks, are set forth in the Company's 8-K filed with the SEC on February 11, 2021. The Company expressly disclaims any obligation to update or reaffirm these expectations. Near-Term Expected Revenue Drivers Expected 2021 Net Sales: Full indication and prescribing information for VIVITROL may be found at www.vivitrol.com Full prescribing information for ARISTADA may be found at www.aristada.com

Capital Allocation: Focused on Organizational Efficiency and Cost Management

Conducted Comprehensive Analysis of Operations and Cost Structure and Implemented Significant Changes Conducted extensive review of operations and cost structure both internally and with external advisors to identify potential areas for efficiencies and savings

Completed peer benchmarking to identify areas of focus for potential saving initiatives

Assessed resource requirements across multiple functions to determine optimal in-house/outsourcing strategy

Evaluated enterprise system/processes that could drive long-term efficiencies

Commercial infrastructure reorganization

Addiction salesforce refocused to increase emphasis on alcohol dependence

Psychiatry infrastructure realigned to drive efficiencies and reallocate resources to support potential launch of LYBALVI

Prioritized highest-ROI development programs

Determined optimal balance of internal/external resources

Identified efficiency opportunities; implementation ongoing

Comprehensive Cost Structure Review Optimization Initiatives Undertaken

Disciplined Capital Allocation Supports Highest ROI Priorities Drive operating margins of commercial business and focus on profitability Create value through innovation and position ALKS for future growth

Disciplined Capital Allocation Supports Highest-ROI Priorities VIVITROL

Awareness and marketing campaign for alcohol dependence indication

Addiction sales infrastructure

Drive growth through continued focus on two-month dose plus ARISTADA INITIO

Psychiatry sales infrastructure Shared commercial services support

Market Access and key account management infrastructure

Patient access services

Digital marketing capabilities