An occasional drink is unlikely to be dangerous with doxycycline, but health authorities say it's best not to drink alcohol while you're on it, because alcohol can make the antibiotic less effective, especially if you drink heavily or regularly.
This is not the violent, disulfiram-type reaction you get mixing alcohol with metronidazole; with doxycycline the main concern is that alcohol can undercut how well the antibiotic works. The NHS's own doxycycline page states it plainly: "You can eat as normal while taking doxycycline, but it's best not to drink alcohol because it can stop the medicine working properly." The NHS antibiotics-interactions page lists doxycycline among medicines where "it may be best to avoid drinking alcohol," noting that "alcohol can affect this medicine, and it may also be less effective in people with a history of drinking heavily." Doxycycline may also not be suitable for people who have problems with alcohol dependence or a liver condition, per the same NHS page. For a genuinely one-off drink in an otherwise healthy person the risk is low, but because the downside is a failed course of antibiotics, the source-consistent advice is to skip alcohol until you finish the course. MedlinePlus reinforces why this matters: doxycycline works by "stopping bacteria from growing and spreading," and it stresses taking the full course "even if you feel well."
Who should truly avoid it: anyone with a history of heavy or regular drinking, alcohol dependence, or a known liver condition — the NHS flags these as reasons doxycycline may not be suitable, and heavy drinking can make it less effective. Timing/separation: the sources give NO specific "wait X hours" alcohol window for doxycycline (unlike the defined 48-hour rule for metronidazole) — the safest approach is to avoid alcohol for the whole course, not to time it around doses; do not invent an hours-based rule. Don't confuse this with doxycycline's real food-timing rules: it does need to be separated by a few hours from antacids, calcium supplements and iron (take antacids/calcium 1–2 hours apart and iron ~2–3 hours apart per MedlinePlus) — but that's about mineral binding, not alcohol, and milk/food does not notably affect doxycycline absorption. Call a clinician if your infection isn't improving or is getting worse (a sign the drug may not be working), or if you notice possible liver trouble: yellowing skin or eyes, dark urine, severe upper-right abdominal pain, or unusual tiredness. Bottom line: an occasional single drink is unlikely to be harmful, but the recommended, source-backed answer is to avoid alcohol until you've finished the course.
This is general reference, not medical advice, and not a guarantee of safety. Interactions depend on your doses, health conditions, and other medicines. Always confirm with your pharmacist or doctor before combining products, and follow the dosing on each label.