The effects and duration of action depend on the specific synthetic opioid, dosage, frequency of use, set (person) and setting (environment). In addition to analgesic properties, the effects include deep, comprehensive relaxation, drowsiness, slowed breathing, constricted pupils and euphoria. However, the euphoric effect is not as pronounced as that of heroin.⁽¹⁾ There are no reliable findings on many of these opioids. The available findings come almost exclusively from user reports and are described as comparable to those of natural or semi-synthetic opioids.

The most common negative effects include nausea, dizziness, vomiting, exhaustion and headaches. Substances from the group of new synthetic opioids differ greatly in their potency, which is often a multiple of up to a thousand or ten thousand times that of morphine. This makes it difficult or even impossible to find a dosage that leads to the desired effects. There is a risk of consuming too high/toxic a dose, which can result in life-threatening respiratory depression.

Regular use can lead to the development of a tolerance to the substance and a higher dose must be taken to achieve the same effect. Synthetic opioids (like natural opiates) can also lead to physical (i.e. bodily) dependence and a decrease in libido (sensation of pleasure) after prolonged use.

During withdrawal, the first physical symptoms occur within 2-24 hours of the last use, depending on the rate at which the substance is broken down, the frequency of use and the dosage. They reach their peak after 1-3 days and usually subside after approx. 1-2 weeks.^{(2, 3)} Symptoms include sweating, anxiety, diarrhoea, cramps, aching limbs, shivering, trembling and goose bumps. Withdrawal should always take place under medical supervision.

As with all opioids, serious interactions can occur through joint use with cocaine (exposure due to opposite effects → cardiovascular failure) and downers such as heroin, alcohol, GHB/GBL or benzodiazepines.⁽⁴⁾ However, the combination of opioids with benzodiazepines is relatively common. There is evidence that this combination leads to increased euphoria at moderate doses. However, the risk of an opioid overdose and associated respiratory depression is greatly increased.⁽⁵⁾

One of the most widely used synthetic opioids is fentanyl due to its use in pain therapy. Forms of application include pain patches, tablets, lollipops and nasal sprays. Pain patches in particular are consumed in part due to the lack of availability of heroin, in which they are split and boiled so that the active ingredient can be injected. However, as the active ingredient is not evenly distributed on the patch, this type of consumption is very risky and has led to several overdoses in the past (sometimes resulting in death).^{(6,7, 8)} In addition to the active ingredient, other substances (such as silicone oils and surfactants) can also be dissolved during boiling, which can be life-threatening if injected.

Depending on the type of application, the active ingredient enters the body via the blood or the mucous membranes (or the skin in the case of pain patches). The onset of action occurs after approx. 10 seconds with intravenous consumption, after approx. 2-5 minutes with nasal consumption and after approx. 1-2 hours with pain patches on the skin. The best-known opioid receptors are the G-protein-coupled μ-, κ- and δ- Opioid receptors. According to the current state of research, most synthetic opioids act as µ-opioid receptor agonists (e.g. fentanyl and its derivatives, U47700). In higher doses, the opioid AH-7921 also acts as an agonist at the κ-opioid receptor, MT-45 stimulates, among other things, the δ- and κ-opioid receptors.⁽¹⁾ The activation of the opioid receptors results in a pain-relieving effect (analgesia). Certain receptor types are particularly responsible for other possible effects. Activation of the μ-Opioid receptor causes euphoria, respiratory depression, sedation (severe drowsiness) and nausea. The κ-Receptor binding, on the other hand, can lead to dysphoria (emotional moodiness) and sedation. Activation of the δ- opioid receptor is associated with cramps, a sense of reward and anxiety relief.⁽⁹⁾ Therefore, the effects produced are highly dependent on the receptor binding profile of the opioid in question. Fentanyls are generally strong μ-Opioid receptor agonists.

• Diseases of the lungs (e.g. asthma), as synthetic opioids depress the respiratory centre.
• Pregnancy and breastfeeding

• If you have a synthetic opioid: Be sure to use a drug checking service to make sure which substance it is.
• Use an (accurate) microgram scale.
• Pay attention to the exact dosage: the analgesic potency of the substances varies greatly (U47700: approx. 7.5 times as strong as morphine, fentanyl: approx. 100 times, carfentanil up to approx. 10,000 times).
• Use gloves and a mask when handling, as well as a pad that you can clean or dispose of afterwards.
• Only use pharmaceutical products (e.g. fentanyl patches) in accordance with the instructions on the package leaflet.

Risks exist not only for potential consumers, but also for people involved in transport and production. For example, the highly potent synthetic substances pose health risks to postal workers and law enforcement officers.

Many synthetic opioids are subject to the Narcotic Drugs Act. For example, fentanyl and its derivatives. In particular, the acquisition, production, import and export, transfer to and procurement for others (transfer, sale, etc.) is punishable by law and can result in fines and imprisonment.

Other substances are subject to the New Psychoactive Substances Act (NPSG). Any person who intentionally produces, imports, exports or transfers or procures new psychoactive substances to be used by the other person or a third party to achieve a psychoactive effect in the human body is liable to prosecution.

(1) Zawilska, J. B. (2017). An expanding world of novel psychoactive substances: opioids. Frontiers in psychiatry, 8, 110.

(2) Link  (Status: 02/2018)

(3) Link (Status: 02/2018)

(4) Jones, J. D., Mogali, S., & Comer, S. D. (2012). Polydrug abuse: A review of opioid and benzodiazepine combination use. Drug and Alcohol Dependence, 125, 8-18.

(5) Sun Eric C, Dixit Anjali, Humphreys Keith, Darnall Beth D, Baker Laurence C, Mackey Sean et al. Association between concurrent use of prescription opioids and benzodiazepines and overdose: retrospective analysis BMJ 2017; 356 :j760

(6) Plörer, D, Martin, G, Winter, C, Koller, G, Walcher, S, Muselmann, R, Schäfer, F, Al-Iassin, J, Pogarell, O (2012): Fentanyl abuse in opiate-dependent patients with and without opiate substitution therapy (methadone, polamidone, buprenorphine). Addiction Medicine in Research and Practice 14 (4) 18

(7) Bavarian Academy for Addiction and Health Issues (2017): Prevention of drug-related deaths. Facts, figures and practical examples, 2nd revised and supplemented edition, January 2017

(8) Link

(9) Stein, C. (2016). Opioid receptors. Annual review of medicine, 67, 433-451.