Inclusion Criteria:

* Male or female subject aged ≥ 18 years.
* Confirmed diagnosis of CLL or small lymphocytic leukemia (SLL) per iwCLL 2018 criteria.
* History of ≥1 non-melanoma skin cancer (NMSC) diagnosed within the last 5 years
* Adequate liver function as defined as:

* Total Bilirubin ≤ 1.5x institutional upper limit of normal (ULN)

---Subjects with a known diagnosis of Gilbert's Syndrome: direct bilirubin ≤ 1.5x ULN
* AST(SGOT)/ALT(SGPT) ≤ 3 × institutional ULN
* For female subjects: Negative pregnancy test or evidence of post-menopausal status. The post-menopausal status will be defined as having been amenorrheic for 12 months without an alternative medical cause. The following age-specific requirements apply:

--Women \< 50 years of age:
* Amenorrheic for ≥ 12 months following cessation of exogenous hormonal treatments; and
* Luteinizing hormone and follicle-stimulating hormone levels in the post-menopausal range for the institution; or
* Underwent surgical sterilization (bilateral oophorectomy or hysterectomy).
* Women ≥ 50 years of age:

* Amenorrheic for 12 months or more following cessation of all exogenous hormonal treatments; or
* Had radiation-induced menopause with last menses \>1 year ago; or
* Had chemotherapy-induced menopause with last menses \>1 year ago; or
* Underwent surgical sterilization (bilateral oophorectomy, bilateral salpingectomy, or hysterectomy).
* Female subjects of childbearing potential and male subjects with a sexual partner of childbearing potential must agree to use a highly effective method of contraception as described in Section 5.4.1.

Exclusion Criteria:

* Received cytotoxic chemotherapy (including fludarabine, cyclophosphamide, bendamustine, or chlorambucil) within the last 6 months
* Received allogeneic stem cell transplant within the last 6 months.
* Taking nicotinamide or niacin supplements within the last 4 weeks.
* Taken acitretin or other oral retinoids within the past 6 months
* Received field treatment for AKs (topical use of 5-fluorouracil, imiquimod, diclofenac, retinoids; topical photodynamic therapy for AKs; laser resurfacing or chemical peel treatments for AKs) within the previous 4 weeks
* Large areas of confluent skin cancer at baseline preventing accurate assessment and counting of individual new skin cancers
* Need for ongoing carbamazepine use (possible interaction with nicotinamide)
* Severe GI malabsorption that may interfere with absorption of nicotinamide (per investigator's discretion)
* Patients with an expected life expectancy \< 2 years
* Current evidence of uncontrolled, diabetes.
* Current evidence or history of peptic ulcer disease.
* Known HIV infection with a detectable viral load within 6 months of the anticipated start of treatment.

Note: Subjects on effective antiretroviral therapy with an undetectable viral load within 6 months of the anticipated start of treatment are eligible for this trial.

* Known active uncontrolled infection.
* Medical, psychiatric, cognitive, or other conditions that may compromise the subject's ability to understand the subject information, give informed consent, comply with the study protocol or complete the study.
* Known prior severe hypersensitivity to investigational product (IP) or any component in its formulations (NCI CTCAE v5.0 Grade ≥ 3).
* Subjects taking prohibited medications as described in Section 6.7.1. A washout period of prohibited medications for a period of at least five half-lives or as clinically indicated should occur before the start of treatment.
* Have ever received a solid organ transplant and are currently taking immunosuppressive medications.