13G Filing: Perceptive Advisors and Avadel Pharmaceuticals PLC (AVDL)

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No.     )*

Avadel
Pharmaceuticals plc

(Name of Issuer)

Ordinary
shares, nominal value $0.01 per share

(Title of Class of Securities)

05337M 10 4¹

(CUSIP Number)

November 1, 2017

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to
designate the rule pursuant to which this Schedule is filed:

The information required in the remainder of this cover
page shall not be deemed to be filed for the purpose of Section 18 of the Securities Exchange Act of 1934 (Act) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other
provisions of the Act (however, see the Notes).

Item 1(a). Name of Issuer:

Avadel Pharmaceuticals plc (the Issuer)

Item 1(b). Address of Issuers Principal Executive Offices:

Block 10-1

Blanchardstown
Corporate Park, Ballycoolin

Dublin 15, Ireland

Item 2(a). Names of Persons Filing:

The names of the persons filing this report (collectively, the Reporting Persons) are:

Perceptive Advisors LLC (Perceptive Advisors)

Joseph Edelman (Mr. Edelman)

Perceptive Life Sciences Master Fund, Ltd. (the Master Fund)

Item 2(b). Address of Principal Business Office or, if None, Residence:

The address of the principal business office of each of the Reporting Persons is:

51 Astor Place, 10th Floor

New
York, NY 10003

Item 2(c). Citizenship:

Perceptive Advisors is a Delaware limited liability company

Mr. Edelman is a United States citizen

The Master Fund is a Cayman Islands corporation

Item 2(d). Title of Class of Securities:

Ordinary shares, nominal value $0.01 per share. (Common Stock)

Item 2(e). CUSIP Number:

05337M 10 4, which represents the CUSIP number for the American Depositary Shares, each representing one share of Common Stock
(ADRs).

Item 3.     If this statement is filed pursuant to §§ 240.13d-1(b) or 240.13d-2(b) or (c),
check whether the person filing is a:

Not applicable.

Item 4.     Ownership.

The information required by this item with respect to each Reporting Person is set forth in Rows 5 through 9 and 11 of the cover page to this
Schedule 13G. The ownership percentages reported are based on 41,435,000 outstanding shares of Common Stock, as reported in the Issuers Form 10-Q filed on August 9, 2017.

The Master Fund directly holds 2,241,841 ADRs. Perceptive Advisors serves as the investment
manager to the Master Fund and may be deemed to beneficially own the securities directly held by the Master Fund. Mr. Edelman is the managing member of Perceptive Advisors and may be deemed to beneficially own the securities directly held by
the Master Fund.

Item 5.   Ownership of Five Percent or Less of a Class.

If this statement is being filed to report the fact that as of the date hereof the Reporting Persons have ceased to be the beneficial owner of
more than five percent of the class of securities, check the following ☐.

Item 6.   Ownership of More than Five Percent on
Behalf of Another Person.

Not applicable.

Item 7.   Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the

               Parent Holding Company or Control Person.

Not applicable.

Item 8.
Identification and Classification of Members of the Group.

Not applicable.

Item 9.   Notice of Dissolution of Group.

Not applicable.

Item 10.
Certification.

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were
not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that
purpose or effect, other than activities solely in connection with a nomination under § 240.14a-11.

SIGNATURE

After reasonable inquiry and to the best of its knowledge and belief, each of the undersigned certifies that the information set forth in this
statement is true, complete and correct.

Date: November 9, 2017

/s/ Joseph Edelman

JOSEPH EDELMAN

EXHIBIT 1

AGREEMENT

The persons below hereby agree that
the Schedule 13G to which this agreement is attached as an exhibit, as well as all future amendments to such Schedule 13G, shall be filed on behalf of each of them. This agreement is intended to satisfy the requirements of Rule 13d-1(k)(1)(iii)
under the Securities Exchange Act of 1934.

Date: November 9, 2017

/s/ Joseph Edelman

JOSEPH EDELMAN