Stephen at Superwild · 15 May 2026 · 7 min read

Apoquel side effects — what UK dog owners should actually watch for

Apoquel (oclacitinib) is one of the most-prescribed dog medicines in the UK. It works fast, it works well, and most dogs tolerate it without drama. But it is a JAK inhibitor — a class of drug that modulates the immune system — and the patient information leaflet runs to several pages of cautions. Here is what to actually pay attention to, in priority order, from a UK vet's perspective.

The common stuff (mostly tolerable)

Roughly 5–10% of dogs on Apoquel get one of the following, usually in the first month:

• Vomiting — give with food to take the edge off.
• Diarrhoea or soft stools — typically settles within a week.
• Lethargy — most owners report this lifts after 7–14 days.
• Reduced appetite — same window.

If any of these persist past two weeks, your vet should review the dose. The maintenance dose is half the loading dose; sometimes the timing slip is the problem.

The important stuff (call the vet, do not ignore)

New lumps or skin growths

Apoquel suppresses parts of the immune response. Pre-existing tumours can grow faster on it, and new lumps deserve faster veterinary attention than they would in an off-drug dog. Run your hands over your dog once a week. Anything new, firm, or growing — book a consult. Not because Apoquel caused the lump (it usually did not) but because you want a benign verdict, or a fast referral, sooner.

Worsening infections

Existing bacterial and fungal infections (skin, ear, urinary) can flare or progress while on Apoquel because the immune signalling that would otherwise rein them in is dampened. If your dog has had a chronic ear or skin infection, it should be fully resolved before starting Apoquel, and re-checked at the next routine visit.

Demodex in young dogs

Apoquel is not licensed for dogs under 12 months. The reason: demodectic mange — the opportunistic skin mite — can erupt in dogs whose immune systems are still developing, and Apoquel makes it worse before anyone clocks what is happening. Stick to the age licence.

The uncertain lymphoma signal

Some retrospective studies have noted a slight statistical association between long-term oclacitinib use and lymphoma. The current consensus is that the signal is small, confounded (allergic dogs may already be at higher baseline risk), and not a reason to stop the drug for the average patient. It is a reason to maintain regular check-ups and to escalate any new lumps quickly.

The rare stuff (worth knowing about)

Cardiac, reproductive and neurological adverse events have been reported but are uncommon. Cytopenias (low platelet or white-cell counts) are an occasional finding on routine bloods. None of these warrant baseline panic, but they are why baseline and periodic bloodwork is a good idea — see below.

Two questions to ask your vet at the next consult

1. "Can we do baseline bloods before continuing past 6 months?" A sensible vet will agree. CBC + biochemistry every 6–12 months on a long-term Apoquel patient is good practice and catches the rare stuff early.
2. "Are we on the lowest effective dose?" Many dogs stay on twice-daily (the loading dose) for too long. Once skin is settled, once-daily is the maintenance target. Some dogs need every-other-day in steady state.

When to come off Apoquel

Hardly anyone needs to come off Apoquel forever, but reasons to pause or stop include: a new tumour diagnosis, an unresolved infection that needs the immune response back, a planned vaccination cycle (your vet may pause briefly), and pregnancy or breeding (it is not licensed for breeding dogs). There is no withdrawal — you stop, the itch returns within a few days. Have a plan for what bridges the gap (a short steroid course, or switching to Cytopoint, or trialling Atopica).

The cost piece, while we're here

Apoquel is also the medicine where the vet-vs-online price gap is most painful for the average UK owner. For a 15 kg dog on the 5.4 mg strength:

• Average UK vet practice: ~£92/month
• Cheapest VMD-approved online pharmacy: ~£34/month
• Annual saving, net of the £21 CMA-capped prescription fee: ~£675

The price difference does not get worse because of side effects — that is a clinical conversation. But it is worth knowing that the saving funds private blood-test panels and proper review consults, which is exactly the monitoring that catches the rare stuff. Run the numbers on the per-weight calculator or read the prescription guide for the email template.

Not medical advice. Always follow your prescribing vet's instructions. Sources: Apoquel SPC (datasheet) at vmd.defra.gov.uk; Zoetis published safety data; UK peer-reviewed retrospective studies 2019–2024.