Review Finds Meth and Fentanyl Together May Amplify Addiction

Methamphetamine and fentanyl, taken together, may be more rewarding and more relapse-prone than either drug produces on its own, according to a review published October 20 in Frontiers in Behavioral Neuroscience. The authors, four psychologists at the University of New Mexico, conclude that the two drugs interact at the mu-opioid receptor in ways that go beyond simple addition, and they say the direct evidence base for the pairing is far thinner than the scale of the problem.

The review's most specific claim comes from rodent experiments. In one conditioned place-preference study, doses of methamphetamine and fentanyl that produced no preference when given separately produced both preference and reinstatement when administered together. In concurrent-access studies, rats trained on both drugs later showed cross-reinstatement: a cue or priming dose associated with one drug restarted seeking of the other, in both directions.

What the animal work shows

Rodents escalate methamphetamine intake across sessions, develop persistent preference for drug-paired environments, and resume seeking after exposure to cues, stressors or a priming injection. Fentanyl produces a comparable profile — rapid acquisition of place preference, escalating self-administration, reinstatement after extinction. None of that is new, and the review does not present it as new.

The contribution is the combination. In one self-administration experiment described in the paper, rats did not shorten their interval between fentanyl infusions as doses rose the way they do with fentanyl alone, and total intake ended up higher. The authors read that as a change in the relative reinforcing value of each drug under conditions of joint access, not as a simple sum of two separate habits.

The neurochemistry points the same direction without settling the question. Methamphetamine-fentanyl self-administration produced larger reductions in dopamine release and reuptake in the nucleus accumbens than methamphetamine alone, with a more pronounced hypodopaminergic state in males. Those measurements were taken roughly 22 hours after the final session — outside acute drug effects — which the authors treat as evidence of compensatory adaptation after repeated exposure rather than an immediate response to the drug.

Why fentanyl and meth are meeting in the same syringe

The epidemiological backdrop the review assembles is more solid than its mechanistic conclusions. An estimated 2.4 million Americans aged 12 or older reported past-year methamphetamine use in 2024, according to the 2025 National Survey on Drug Use and Health. Self-reported concurrent methamphetamine and fentanyl use nearly doubled between 2011 and 2017. Fentanyl positivity among methamphetamine-positive urine samples climbed from 0.9 percent to 7.9 percent between 2013 and 2018.

The review's framing of that last number is the one that matters for anyone who uses stimulants: much of the overlap is involuntary. Methamphetamine is increasingly cut with fentanyl, so a person who believes they are using a stimulant can be exposed to a potent opioid without knowing it. The result is a population with stimulant tolerance and no opioid tolerance, which is the profile that turns a small exposure into a fatal one.

Globally, roughly 31 million people aged 15 to 64 used amphetamines in 2023, making the class the third most widely used category of controlled substances behind cannabis and opioids, according to the United Nations Office on Drugs and Crime.

The receptor that ties the two together

The review's organizing idea is that mu-opioid receptor agonism modulates methamphetamine's behavioral effects, not only fentanyl's. In the ventral tegmental area, mu-opioid activation on inhibitory GABAergic neurons disinhibits dopamine cells, which is a long-established mechanism for opioid reward. The authors argue the same system shapes conditioned place preference, cue reactivity and behavioral sensitization when methamphetamine is in the picture, which is why blocking or activating the receptor changes stimulant behavior in animal models.

Pharmacokinetics widen the window. Methamphetamine is highly lipophilic, enters the brain quickly and has a half-life of roughly 12 hours in the pharmacological literature cited by the review. Fentanyl's is three to seven hours. A person using both is not alternating between two short experiences; the stimulant is still present while the opioid wears off and is re-dosed.

What the review cannot say

The authors are unusually direct about the limits. Direct research on this specific pairing remains limited, and most of the proposed mechanism is inferred from studies of each drug alone or of methamphetamine combined with other opioids. The behavioral evidence comes from rodents. A rat experiment cannot establish how the same processes work in human relapse, and the review says so.

That leaves clinicians with a plausible mechanism, a strong epidemiological trend and no trial evidence. It also leaves a gap the authors name explicitly: the neurobiological and behavioral consequences of concurrent methamphetamine-fentanyl use have been largely unexamined, even as the combination has become common.

What it means at the treatment door

The practical implication runs through screening rather than pharmacology. Programs that treat stimulant use disorder and opioid use disorder as separate tracks will miss people who use both, which is the population the urine data describe. Polysubstance use of this shape also complicates withdrawal management: opioid tolerance can be low while stimulant tolerance is high, and the danger during a first week of abstinence is not only craving but cardiovascular and psychiatric instability.

There is no approved medication for methamphetamine use disorder, and the review does not propose one. What it offers is an argument for asking every patient about both drugs, and for treating a positive fentanyl test in someone who came in for meth as information about supply rather than a separate diagnosis. Anyone who uses stimulants and cannot be sure what is in the bag can carry naloxone; a reversal agent does not treat the stimulant, but it buys the time an overdose requires.