Access to Substance Abuse Treatment Act of 2018 This bill amends the Public Health Service Act to allow the Substance Abuse and Mental Health Services Administration to make grants to: (1) increase the availability of treatment for abuse of opioids, cocaine, methamphetamine, 3,4-methylenedioxymethamphetamine (ecstasy), and phencyclidine (PCP); (2) provide vouchers to individuals in underserved populations for services related to such treatment; and (3) establish programs to provide for and coordinate the provision of specified services to individuals reentering the community after successfully receiving treatment for abuse of such substances. The grant program to provide residential substance abuse treatment to pregnant and postpartum women is revised to: (1) make caregiver parents eligible for treatment, (2) make Indian tribes and tribal organizations eligible for grants, and (3) set forth the priority for allocation of grants. The National Institute on Drug Abuse must conduct research on the effectiveness of drugs to reduce the problems associated with stimulant abuse. The Department of Health and Human Services must seek to enter into a contract with the National Academy of Medicine (formerly known as the Institute of Medicine) to complete a literature review on the effectiveness of drugs for the treatment of stimulant abuse. The Government Accountability Office must study: (1) the impact of the programs authorized by this Act, (2) how the level of federal funding available for such treatment compares to the amount necessary to provide adequate treatment, and (3) the cost savings of effective treatment due to the reduced need for criminal justice and other services.