10 Scariest Prescription Drug Side Effects

Adverse drug reactions send an estimated 4.5 million Americans to a physician's office or emergency room every year.¹

Here are 10 of the most serious, potentially life-threatening side effects linked to commonly prescribed medications—and the drug classes most associated with each.¹

Hallucinations

Hallucinations occur when a person senses something that isn't actually present—visual, auditory, olfactory, tactile, or gustatory. They're a common symptom of schizophrenia but can also result from excessive alcohol intake, drug abuse, depression, sleep deprivation, dementia, or certain prescription medications.²

A number of psychiatric medications—olanzapine (Zyprexa), quetiapine (Seroquel), and haloperidol (Haldol)—have been associated with causing hallucinations, along with zolpidem (Ambien), eszopiclone (Lunesta), clonazepam (Klonopin), lorazepam (Ativan), ropinirole (Requip), and some seizure medications.³ Cephalosporins and sulfa drugs—2 common antibiotic classes—have also been associated with hallucinations in rare cases.

Memory Loss

Memory loss is a natural part of aging, but it can also be a side effect of certain medications.

The most notable class linked to memory loss is nonbenzodiazepine sedative hypnotics—Ambien, Lunesta, and Sonata (zaleplon). In 2019, the FDA added a boxed warning—its strongest safety warning—to all 3 after identifying 66 cases over 26 years of "complex sleep behaviors" (such as sleepwalking and sleep-driving) resulting in serious injury or death.⁴ These events are frequently associated with no memory of the episode afterward. Other medications linked to memory loss include benzodiazepines, statins, certain seizure medications, opioids, and incontinence drugs.

Priapism

Priapism is a painful, persistent erection unrelated to sexual stimulation. Left untreated, it can cause tissue damage and permanent inability to get or maintain an erection.

Causes include certain medical conditions, trauma, alcohol use, and prescription medications—including trazodone (Desyrel), clozapine (Clozaril), hydroxyzine (Atarax), chlorpromazine (Thorazine), prazosin (Minipress), warfarin (Coumadin), testosterone therapy, and SSRIs such as sertraline (Zoloft), fluoxetine (Prozac), and paroxetine (Paxil).⁵

Blood Clots

Blood clots form under many normal circumstances in response to injury, but some medications raise the risk of clot formation. Left untreated, a clot can break loose and travel elsewhere in the body—sometimes fatally.

Oral contraceptives and hormone therapy drugs all carry an FDA-required warning for increased blood clot risk, as do all testosterone replacement products.¹¹

Compulsive Behaviors

Compulsive behavior means repeatedly performing an act without control, interfering with daily life.

Ropinirole (brand name Requip has been discontinued in the US; generic ropinirole remains available) and pramipexole (Mirapex)—dopamine agonists for Parkinson disease and restless legs syndrome—can cause impulse-control problems. Per current prescribing information, patients may experience new or increased gambling urges, sexual urges, uncontrolled spending, binge or compulsive eating, or other urges while on treatment. ³ Carbidopa-levodopa (Sinemet) carries the same warning, and the antipsychotic aripiprazole (Abilify) has more recently been linked to compulsive gambling in some patients.

Stevens-Johnson Syndrome

Stevens-Johnson syndrome (SJS) is a rare, life-threatening hypersensitivity reaction of the skin and mucous membranes, causing rapid blistering, necrosis, and skin shedding.⁶

Lamotrigine (Lamictal) carries a relatively high SJS incidence, especially at high starting doses—prompting an FDA boxed warning. Other associated medications include allopurinol (Zyloprim), acetaminophen (Tylenol), ibuprofen (Motrin), naproxen (Aleve), sulfa drugs, penicillin, barbiturates, and other anticonvulsants.⁷

Birth Defects

An estimated 1 in 33 babies in the US is born with a birth defect.⁸

Thalidomide remains one of the best-known teratogenic drugs in medical history—marketed in the 1950s for insomnia, coughs, colds, and headaches, it caused the deaths of roughly 2,000 children and serious birth defects in more than 10,000 more.¹² Known teratogenic prescription medications include warfarin, divalproex (Depakote), paroxetine (Paxil), topiramate (Topamax), methotrexate (Rheumatrex), ACE inhibitors, ARBs, NSAIDs, oral contraceptives, statins, and tetracyclines.¹² Risk is highest in the first trimester, but patients should discuss medication use with their care team at every stage of pregnancy.

Cancer

Tamoxifen (Nolvadex), used to treat breast cancer, carries an FDA boxed warning for increased risk of uterine cancer (endometrial adenocarcinoma and uterine sarcoma), based on data from clinical trials.¹³

Pioglitazone (Actos), a type 2 diabetes medication, carries a warning about increased bladder cancer risk based on observational study data.¹³ All GLP-1 receptor agonists carry a boxed warning for thyroid C-cell tumor risk observed in animal studies,¹³ and metronidazole (Flagyl) has shown carcinogenic effects in animal studies as well.

Suicidality

Since 2004, the FDA has required a boxed warning on all antidepressants noting an increased risk of suicidal thinking and behavior in children and adolescents. In 2007, the FDA expanded that warning to include young adults ages 18–24, based on age-stratified analysis of clinical trial data.⁹ The same expanded labeling notes no evidence of increased risk in adults over 24, and a protective effect in adults 65 and older.

Clinical evidence on the topic continues to evolve — some studies show a correlation between antidepressant use and increased suicidal ideation, while others show no increased risk. Other medications associated with suicidal thinking and behavior include montelukast (Singulair), isotretinoin (Claravis), varenicline (Chantix), and mefloquine (Lariam).

Death

Sudden cardiac death is the leading cause of natural death in the US, with an estimated 325,000 adult deaths annually.

Antipsychotics including quetiapine (Seroquel), olanzapine (Zyprexa), and risperidone (Risperdal) have been associated with sudden cardiac death, with higher incidence in elderly patients and those on concurrent cardiac medications.¹⁰ Antiarrhythmic agents carrying this warning in their package inserts include sotalol (Betapace), amiodarone (Cordarone), and procainamide (Procanbid).¹⁴ Morphine and Adderall labeling also warns of increased sudden death risk related to cardiac abnormalities.¹⁴

References

1. Sultana J, Cutroneo P, Trifirò G. Clinical and economic burden of adverse drug reactions. J Pharmacol Pharmacother. 2013;4(Suppl1):S73-S77. doi:10.4103/0976-500X.120957

2. Wade M. Medication-related visual hallucinations: what you need to know. EyeNet Magazine (American Academy of Ophthalmology). March 2015. Accessed September 3, 2026.

3. Ropinirole (Requip, Requip XL) [prescribing information]. Note: Requip brand name discontinued in the US; generic ropinirole remains available. Current full prescribing information available via FDA at accessdata.fda.gov/drugsatfda_docs.

4. U.S. Food and Drug Administration. FDA adds Boxed Warning for risk of serious injuries caused by sleepwalking, sleep driving, and other activities while not fully awake with certain prescription insomnia medicines. FDA Drug Safety Communication. Published April 30, 2019.

5. UCSF Medical Center. Drugs reported to cause priapism.

6. Rehmus W. Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN). Merck Manual. Accessed September 3, 2026.

7. Stevens-Johnson Syndrome Foundation. List of medications associated with Stevens-Johnson syndrome and toxic epidermal necrolysis. Confirmed active as of this update: sjsupport.org.

8. MedlinePlus. Birth defects. U.S. National Library of Medicine. Accessed September 3, 2026.

9. Friedman RA. Expanding the black box—depression, antidepressants, and the risk of suicide. N Engl J Med. 2007;356(23):2343-2346. doi:10.1056/NEJMp078015

10. Sicouri S, Antzelevitch C. Sudden cardiac death secondary to antidepressant and antipsychotic drugs. Expert Opin Drug Saf. 2008;7(2):181-194. doi:10.1517/14740338.7.2.181 [Retained — reasonably current given rarity of newer consolidated reviews on this exact cross-drug-class topic; a pharmacist may know of a more recent one.]

11. U.S. Food and Drug Administration prescribing information. Combined hormonal contraceptives, hormone therapy, and testosterone replacement products all carry class-labeled warnings for increased venous thromboembolism risk. Verify current label for each specific product via DailyMed (dailymed.nlm.nih.gov) before publishing — this is a class-level citation, not a single-product one, so a pharmacist should confirm it reads accurately for the specific products named.

12. Thalidomide and other teratogenic medications — verify against a current, per-drug teratogenicity source (e.g., MotherToBaby, fda.gov pregnancy labeling) before publishing. Not independently verified in this pass — this is the weakest citation in the refresh; the drug list itself is clinically standard, but I did not confirm a single current source covering all of them together the way this paragraph implies. Recommend either sourcing MotherToBaby's fact sheets per drug class, or splitting this into per-drug inline citations.

13. Tamoxifen (Nolvadex) boxed warning confirmed via current FDA/DailyMed label (dailymed.nlm.nih.gov) — verified in this pass. Pioglitazone (Actos) bladder cancer warning and GLP-1 receptor agonist class boxed warning for thyroid C-cell tumors are standard, well-established FDA labeling — not independently re-confirmed link-by-link in this pass; recommend a quick DailyMed check for each before publishing given this section touches cancer risk specifically.

14. Antiarrhythmic (sotalol, amiodarone, procainamide) and morphine/Adderall cardiac-death labeling — standard package-insert content; not independently re-verified in this pass. Recommend a DailyMed spot-check before publishing.