Short answer: In the label's studies of the extended-release tablet, the most reported Trental side effects were dyspepsia (2.8% vs 4.7% on placebo), nausea (2.2% vs 0.8%), dizziness (1.9% vs 3.1%), vomiting (1.2%) and headache (1.2% vs 1.6%), and 3.1% stopped because of side effects. Rates were higher with an older immediate-release capsule. There is no boxed warning; the label warns about bleeding and drug interactions.
What Trental is approved for: Trental is a xanthine-type medicine that lowers blood viscosity (thickness), approved for intermittent claudication (leg pain when walking) caused by chronic occlusive arterial disease of the limbs. The label says it can improve function and symptoms but is not meant to replace more definitive treatment such as bypass surgery or removal of artery blockages (Indications and Usage).
Source
TRENTAL (pentoxifylline) extended-release tablets, prescribing information. Validus Pharmaceuticals. DailyMed, version published October 1, 2026. DailyMed setid b0d1d50f-9023-4e2f-889f-a5febba70fa5
No. The current Trental label has no boxed warning.
These are the label's own numbers from the Adverse Reactions section. Where the label compares with placebo (a dummy treatment), both columns are shown, so you can see how much more often a reaction happened on Trental.
| Side effect | Extended-release tablets: TRENTAL (321) | Tablets: Placebo (128) | Immediate-release capsules: TRENTAL (177) | Capsules: Placebo (138) |
|---|---|---|---|---|
| Discontinued for Side Effect | 3.1 | 0 | 9.6 | 7.2 |
| Angina/Chest Pain | 0.3 | - | 1.1 | 2.2 |
| Arrhythmia/Palpitation | - | - | 1.7 | 0.7 |
| Flushing | - | - | 2.3 | 0.7 |
| Abdominal Discomfort | - | - | 4.0 | 1.4 |
| Belching/Flatus/Bloating | 0.6 | - | 9.0 | 3.6 |
| Diarrhea | - | - | 3.4 | 2.9 |
| Dyspepsia | 2.8 | 4.7 | 9.6 | 2.9 |
| Nausea | 2.2 | 0.8 | 28.8 | 8.7 |
| Vomiting | 1.2 | - | 4.5 | 0.7 |
| Agitation/Nervousness | - | - | 1.7 | 0.7 |
| Dizziness | 1.9 | 3.1 | 11.9 | 4.3 |
| Drowsiness | - | - | 1.1 | 5.8 |
| Headache | 1.2 | 1.6 | 6.2 | 5.8 |
| Insomnia | - | - | 2.3 | 2.2 |
| Tremor | 0.3 | 0.8 | - | - |
| Blurred Vision | - | - | 2.3 | 1.4 |
Drug-related reactions. The extended-release tablet (the marketed form) was studied for up to 60 weeks at 400 mg two or three times a day, outside the U.S.; the immediate-release capsule (used only in controlled trials) for up to 24 weeks at 200 to 400 mg three times a day. A dash means the label shows no rate. Numbers of patients in brackets.
The label says side effects were more common in the capsule studies, with dose-related increases in digestive and nervous system effects, and few patients stopped the extended-release tablet because of side effects.
Reported since marketing or in other trials at less than 1%, with an uncertain link: shortness of breath, swelling, low blood pressure, poor appetite, gallbladder inflammation, constipation, dry mouth, anxiety, confusion, depression, seizures, aseptic meningitis, nosebleed, brittle nails, itching, rash, hives, angioedema and low white cell count.
Rare reports since 1972, with no established link: fast or irregular heartbeat, hepatitis, jaundice, raised liver enzymes, low fibrinogen, low blood counts including aplastic anemia, leukemia, purpura, low platelets, and anaphylactic reactions including shock.
In mild to moderate liver impairment, exposure (AUC) to pentoxifylline rose 6.5-fold after one 400 mg dose. In severe kidney impairment, exposure to Metabolite V rose 12.9-fold on 400 mg three times a day.
In a poison control report of 44 overdoses, symptoms usually began 4 to 5 hours after ingestion and lasted about 12 hours. The highest amount taken was 80 mg/kg, with flushing, low blood pressure, convulsions, drowsiness, loss of consciousness, fever and agitation. All recovered.
The Warnings and Precautions part of the label lists these. Each line gives the label's heading and what it means in plain words.
Who should not take it. People with recent bleeding in the brain or retina, and people who have not tolerated Trental or other methylxanthines such as caffeine, theophylline and theobromine.
Allergic reactions. At the first sign of an anaphylactic or anaphylactoid reaction, Trental is stopped.
Heart symptoms. Occasional angina, low blood pressure and irregular heartbeat have been reported. Controlled trials did not show these more often than on placebo, but as a methylxanthine some people may have them.
Bleeding. Bleeding has been reported with or without NSAIDs, blood thinners or antiplatelet medicines. People on warfarin have prothrombin time checked more often, and people with bleeding risks such as recent surgery, stomach ulcers or brain or retinal bleeding have periodic bleeding checks, including hematocrit or hemoglobin.
Liver and kidney impairment. Exposure to pentoxifylline or its active metabolites is higher in liver or kidney impairment, and the consequences are not known.
Theophylline and CYP1A2 inhibitors. Taking Trental with theophylline raises theophylline levels and has caused theophylline toxicity in some people, so levels are monitored. Strong CYP1A2 inhibitors such as ciprofloxacin or fluvoxamine may raise pentoxifylline levels, and cimetidine raised average levels by about 25%.
Blood pressure medicines. Small drops in blood pressure have been seen, so periodic blood pressure checks are recommended for people on blood pressure medicines, whose dose may be lowered if needed.
Breastfeeding. Pentoxifylline passes into breast milk. Because of tumors seen in rats, a choice is made between stopping nursing and stopping the medicine.
Dose Track has a side-effect log. Each entry records the symptom, a severity (none, mild, moderate, severe or life-threatening), the date and an optional note, and can be tied to a specific dose or logged on its own. The history screen shows what you logged over time, which makes it easier to tell your prescriber when a reaction started and whether it is easing. The app records what you enter; it does not judge whether a symptom is serious. For anything on the label's call-your-doctor list, call your doctor.
This page is what the FDA label says, in plain words. It is not medical advice. Follow your prescriber, and ask them or your pharmacist about anything here.