Cannabis (marijuana) is the most commonly used illicit psychoactive drug in the United States (U.S.) with an estimated 9.6% of the population aged 12 and older reporting use in the past month [1]. States are increasingly legalizing both recreational and medicinal cannabis use [2]. The majority of new users are under 18 years of age [3], and cannabis use has increased among youth and teens since 2007 [4]. In addition, pregnant women are increasingly using it to mitigate morning sickness. In the U.S., between 2002–2003 and 2016–2017, the adjusted prevalence of past-month cannabis use increased from 3.4 to 7.0% among pregnant women overall and from 5.7 to 12.1% among pregnant women during the first trimester [5]. A recent national survey suggested that the public perception of “great risk” from weekly cannabis use has dropped from 50.4% in 2002 to 33.3% in 2014 [6]. Another recent survey found that 81% of U.S. adults believe that cannabis has at least one health benefit, such as use in pain management, disease treatment, or relief of anxiety, stress, or depression. While 91% of U.S. adults also believe cannabis use has at least one risk, including those associated with legal issues, 9% believe it has no risks, and the American public has an overall favorable view of cannabis [7].

Cannabis is composed of over 400 chemicals, of which over 60 are cannabinoid compounds. The four major compounds include Δ⁹-tetrahydrocannabinol (THC), cannabidiol (CBD), Δ⁸-tetrahydrocannabinol, and cannabinol [8]. The major psychoactive cannabinoid in cannabis, THC, targets the endocannabinoid (eCB) system, which regulates biological processes involved in development and neuroplasticity [9]. It mimics eCB action, exerting most of its effects via cannabinoid receptors (CBR)s 1 and 2. CBR1 is one of the most abundant G protein-coupled receptors in the adult brain, and it is localized in regions important in movement, cognition, attention, emotion, and memory [10]. In animals, expression begins early in the central nervous system (CNS) during embryonic development [11–13]. One study found CBR1 expressed in the human fetal brain at 20 weeks, with high expression in the hippocampus and amygdala [14]. In contrast, CBR2 is mainly expressed in immune cells. Male mitotic germ cells also express a high level of CBR2, whose activation promotes their differentiation and progresses spermatogenesis [15••]. During adolescence, the eCB system continues to facilitate neurodevelopment through its involvement in neuroplasticity and synaptic function. Levels of CBRs fluctuate during adolescence and depend on the brain region. For instance, there is a rapid, sustained increase in CBR binding sites in the striatum that is reduced by half in early adulthood [16], as well as high levels in limbic related regions that gradually decrease by adulthood [17]. Tightly regulated biosynthetic pathways ensure proper signaling throughout development, and correct brain function depends heavily on the temporal and spatial layout of the eCB system [18]. Thus, exposure to THC, especially during critical windows of brain development, has the potential to disrupt the tightly regulated system.

Parallel to the increase in adolescent cannabis use, the percentage of adolescents and young adults experiencing certain types of psychiatric disorders has risen in the United States over the past decade, despite the lack of increase in adults [19]. In human studies, THC has been shown to disrupt the development and function of the brain [20, 21], and in animals, THC has been experimentally shown to lead to molecular impairments that are heritable and extend into subsequent generations [22], thus increasing the risk of offspring developing a psychiatric disease [23–26]. Three different routes of multigenerational transmission have been summarized in a prior review [27]; they include fetal programming (direct effect), germline transmission (direct effect), and behavioral or social transfer (indirect effect). The first route is typical for prenatal exposure, the second route is typical for pre-gestational exposure, while the third route is typical for both.

In a recent commentary, which was published in response to a study examining the epigenetic impact of cannabis use on rat and human sperm [28••], the authors highlighted that the epigenomic toxicology of cannabinoids should have priority on the research agenda, especially considering the potential transgenerational health implications [29]. A review published in 2016 focused on the epigenetic effects of cannabis exposure [22]. The authors noted that the majority of addiction-related epigenetic neurobiological studies had targeted the adult brain, while there was a dearth of literature on the potential intergenerational impacts of cannabis [22]. Another article published in 2018 provided an overview of the current data regarding vulnerabilities of the developing brain to cannabinoid exposure during sensitive windows of development, especially with regard to epigenetic changes associated with cannabis use [27]. Since that time, additional studies were published that address research gaps and have the potential to better inform clinical guidelines, preventative policy, and public opinion related to cannabis use during specific time points of the life course.

Heritable molecular impairments include epigenetic modifications, such as DNA methylation, histone modifications, and changes in non-coding RNA (ncRNA), which regulate patterns of gene expression by altering DNA accessibility and chromatin structure [30]. DNA methylation occurs when a methyl group is added at a cytosine nucleotide that precede guanines (CpG dinucleotides), influencing DNA function by activating or repressing transcriptional activity of a gene and by altering chromatin accessibility and remodeling [30]. DNA methylation in the promoter region of a gene usually downregulates its expression, while higher DNA methylation in a gene body may promote expression of a gene. In most instances, DNA methylation represses gene expression by preventing the binding of transcription factors, or recruiting proteins that bind to methylated DNA [31]. Histones are large groups of protein complexes that help DNA condense into chromatin. Histone modifications include methylation and acetylation of lysine residues on histone tails, which affect gene expression by altering chromatin structure and accessibility [31]. In addition, ncRNA, such as micro (mi)RNA and long non-coding (lnc)RNA, control DNA availability and transcription, regulate RNA processing and splicing, and form a scaffold upon which layers of DNA regulation are built [29, 32].

Some epigenetic modifications are passed down to offspring through genomic imprinting (1% of mammalian genes), in which offspring only inherit one working copy of a gene. Imprinted genes are silenced via DNA methylation in either the egg or sperm [33]. Other modifications are passed down when genes escape epigenetic reprogramming, a process that occurs during the formation of primordial germ cells and in the early embryo soon after fertilization [34], in which genomic potential resets and epigenetic memory is erased [35].

In this review, we provide an analysis of the recent literature relating to pre-gestational and prenatal cannabinoid exposure and its effect on genes and molecular pathways. Along with the studies discussed in the review, additional animal studies are summarized in Tables 1 and 2, in which molecular changes are observed in the F0 generation of adolescent brain tissue [36–39, 40•, 41••, 42•].