Dangerous Drug Combinations

 English PDF | More Fact Sheets

What are dangerous drug combinations?

Dangerous drug combinations may include mixing of any of the following:

  • Alcohol

  • Tobacco

  • Illegal drugs

  • Prescription medications

  • Non-prescription medications

  • Natural health products

  • Dietary supplements

Some interactions are minor, but others can be dangerous and possibly life-threatening.


How do drug combinations work?

When drugs are combined, there are three possible effects:

1) They act independently of one another. For example, alcohol does not seem to interfere with vitamins or oral contraceptives (birth control pills), or vice versa.

2) They can increase each other’s effects. This could happen because they affect the brain in the same way or because one drug increases the concentration of the other in the body. For instance, alcohol and antihistamines are both central nervous system depressants. This means they slow the central nervous system down. Mixing them can make both the wanted effects of the drugs (e.g., decreasing self-consciousness) as well as the side-effects (e.g., drowsiness) stronger. Some drugs become more dangerous when they are mixed. This is because their effects are compounded, increasing the way they affect the body and the risk of side effects or overdose.

3) They can decrease each other’s intended effects. This could occur when one drug “blocks” or prevents another drug from working. It could also happen when two drugs have opposite effects on the brain (e.g., drowsiness caused by alcohol versus alertness caused by caffeine). When prescription medication is mixed with other drugs or alcohol, the medication may not work as it should. How common is it for people to use dangerous drug combinations?

People sometimes mix drugs on purpose to make the effects stronger, or to offset unwanted side-effects. Combining heroin and cocaine in a “speedball” is a common example of this.

It is also common for people to use an unsafe combination of drugs without realizing what they are doing. This is widespread among older people who are more likely to be on prescription medications. For example, someone may take sleeping pills after drinking alcohol without understanding that using these drugs together is dangerous. Recent studies show that interactions between alcohol and medication may be a factor for a significant number of the people who are admitted into emergency at hospitals in Canada.


What are the risks of dangerous drug combinations?

Drugs can act together in unexpected ways. It is important to remember this whenever you are taking more than one drug at a time. Awareness helps to lower the risk of dangerous drug interactions, but the ways drugs interact with each other are unpredictable. Sometimes negative effects do not show until a drug has been used for a long time. This applies to therapeutic drugs (like medications), as well as to drugs taken for non-medical purposes.

There is always some danger associated with mixing drugs, but the actual risks involved depend on the kind of drugs being combined and the amounts being used. Alcohol is perhaps most often mixed with other drugs. It is also among the most dangerous when combined with other substances.

Possible results of some drug combinations with alcohol are:


possible result

alcohol + depressants (benzodiazepines such as Valium, Xanax and Ativan)

slow down of central nervous system, loss of coordination, coma, high risk of overdose and death

alcohol + anti-depressants (e.g., Prozac, Elavil, Nardil)

increased central nervous system depression, massive blood pressure increases which can result in brain hemorrhage and death

alcohol + antibiotics (especially Metronidazole, Tinidazole, furazolidone (Furoxone), griseofulvin (Grisactin), Erythromycin, Linezolid, Co-trimoxazole, and Antimalarial Quinacrine (Atabrine))

in general, alcohol may cause antibiotics to be sent out to the body more quickly or take longer to break down in the bloodstream; with the specific drugs listed, combining alcohol may cause headaches, irregular heart rate, low blood pressure, breathlessness, nausea, and vomiting

alcohol + opiates (heroin, codeine)

serious central nervous system depression, which can lead to respiratory arrest (when a person stops breathing) and death

alcohol + tobacco

more than doubles the effect of tobacco, and increases the absorption of toxins

alcohol + cannabis

more than doubles the effect of cannabis, and increases the absorption of toxins

alcohol + stimulants

can mask depressant effects of alcohol. Can increase blood pressure, tension and jitters. When alcohol and cocaine are mixed, the body converts their broken-down products into cocaethylene, a chemical more deadly than cocaine, which increases the risk of overdose

alcohol + prescription medications

cancelled benefits of antibiotics and other medications. Side effects such as cramps, vomiting and headaches (even aspirin can increase the absorption of alcohol and irritates stomach lining. This can lead to gastrointestinal pain and bleeding. Alcohol mixed with anti-alcohol medication can cause headache, nausea, convulsions, coma and death

Some other risky combinations include:

  • Fentanyl and heroin or cocaine: recently flagged as a dangerous drug combination being used on the streets. Fentanyl is 50 to 100 times stronger than morphine. When mixed with heroin or cocaine it can cause irregular heartbeat, the inability to breathe, and death.

  • Warfarin (a blood thinner sold as Coumadin) and Aspirin: can increase the risk of bleeding. The risk is even higher when taken with garlic pills.

  • Lisinopril (a blood pressure drug) and potassium: can cause abnormal heart rhythms. Potassium is often prescribed to rebuild low levels of this mineral caused by some blood pressure drugs.

  • Prescription cholesterol drugs called statins and over-the-counter niacin (a type of vitamin B that also lowers cholesterol): increases the risk of muscle damage.

Care should be taken when illicit drugs are combined with prescription medications as these combinations can be dangerous. The most common toxic effects are:

  • Central nervous system depression, which can range from drowsiness (at its mildest) to coma (at its most severe)

  • Respiratory depression, which can lead to a person not breathing

  • Cardiac effects, such as changes in heart rhythm that can lead to the heart stopping

  • Decreased seizure threshold, meaning that the brain can have a seizure more easily

  • Psychiatric effects, such as psychosis


How to avoid dangerous drug combinations

The only way to completely protect yourself is to avoid mixing drugs altogether. When that is not possible, make sure you know exactly what you are taking, why, and how those drugs could interact with one another.

Many medications can be taken together safely. Talk to your doctor or pharmacist before you start taking anything. Make sure they are aware of the medications you are already on. They will be able to tell you the possible effects of combining specific prescription medications, painkillers or supplements. It is best not to rely on information found on the Internet as it can be unclear or wrong.

People using illegal drugs should also talk to their doctor or pharmacist before taking prescription medications. Illegal drugs can dangerously change the effects of certain medications.


  • Know what you are taking, why and what the possible side effects may be

  • Check with your doctor or pharmacist before mixing drugs or medications of any kind (including dietary supplements and natural health products)


  • Swap prescription medications with friends or family members

  • Increase dosages of medications or pain killers to try to increase the effectiveness


For information on treatment options and resources throughout British Columbia, call Alcohol and Drug Information Referral Service at 1–800–663–1441 (throughout BC) or 604–660–9382 (in Greater Vancouver)

For information on ways to help yourself with a substance use problem, see the “Tips” section of the Here to Help website at www.heretohelp.bc.ca. The website also features detailed information on substances and mental health disorders.

You can also find information on a wide variety of substance use issues on the Centre for Addictions Research of BC website at www.carbc.ca.