What the FDA labels say about taking them together
Procainamide and sulfamethoxazole/trimethoprim
Sulfamethoxazole/trimethoprim's FDA label of December 30, 2024 names procainamide in its drug interactions section; the procainamide label PlainMeds reads does not name sulfamethoxazole/trimethoprim. The sulfamethoxazole/trimethoprim label: “Closely monitor for clinical and ECG signs of procainamide toxicity and/or procainamide plasma concentration if available; Comments: Trimethoprim increases the plasma concentrations of procainamide and its active …”
Check it at the source: the procainamide FDA label on DailyMed (effective February 17, 2025, Caplin Steriles Limited); the sulfamethoxazole/trimethoprim FDA label on DailyMed (effective December 30, 2024, Sun Pharmaceutical Industries, Inc).
What does the procainamide label say about sulfamethoxazole/trimethoprim?
Procainamide's label (effective February 17, 2025) does not name sulfamethoxazole/trimethoprim in these sections. A label lists what its studies and reports found; it is not a complete list.
Source: FDA drug label (Procainamide Hydrochloride), Caplin Steriles Limited, effective February 17, 2025 (SPL set fcaa5e9a-3c8a-4796-aa87-626268b6cf60), via openFDA; the full label on DailyMed
What does the sulfamethoxazole/trimethoprim label say about procainamide?
Sulfamethoxazole/trimethoprim label, drug interactionsDrug (s): Procainamide; Recommendation: Closely monitor for clinical and ECG signs of procainamide toxicity and/or procainamide plasma concentration if available; Comments: Trimethoprim increases the plasma concentrations of procainamide and its active N-acetyl metabolite (NAPA) when trimethoprim and procainamide are coadministered. The increased procainamide and NAPA plasma concentrations that resulted from the pharmacokinetic interaction with trimethoprim are associated with further prolongation of the QTc interval. 10
Source: FDA drug label (Bactrim), Sun Pharmaceutical Industries, Inc, effective December 30, 2024 (SPL set f59d0c04-9c66-4d53-a0e1-cb55570deb62), via openFDA; the full label on DailyMed
These are the labels' own sentences. PlainMeds does not grade interactions; a pharmacist or prescriber can say what they mean for one person's medicines.
Both medicines on the record
| Procainamide | Sulfamethoxazole/trimethoprim | |
|---|---|---|
| Labeled for | n/a | Bacterial infections, Urinary tract infections, Diarrhea |
| Boxed warning | Yes | No |
| Pharmacy cost (NADAC) | n/a | $0.0448 per tablet Sulfamethoxazole-tmp ds tablet |
| Shortage now | No current entry | No current entry |
Source: FDA NDC Directory, CMS NADAC of October 7, 2026 and the FDA Drug Shortage Database (FDA files October 8, 2026)
What next?
- Check more medicines together
- up to five at once