Block and replace vs titration: two ways to take Neo-Mercazole
Block and replace (carbimazole plus levothyroxine) and titration (carbimazole alone, adjusted) compared, with relapse rates, side effects, who each suits, and why block and replace is avoided in pregnancy.
There are two ways to use Neo-Mercazole (carbimazole) for Graves’ disease. Titration uses carbimazole alone, lowering the dose step by step to the smallest amount that keeps you normal. Block and replace uses a fixed, higher dose that shuts the thyroid down completely, plus levothyroxine to supply the hormone your body needs. Both aim for the same 12 to 18 month course.
Key points
- Same relapse rate: 51% with block and replace and 54% with titration in a Cochrane review of 12 trials.
- More side effects with block and replace: more rashes (10% vs 6%) and more people stopping (16% vs 9%).
- Fewer dose changes are the attraction of block and replace for some people, since the dose stays fixed.
- Not in pregnancy: block and replace is avoided because the blocking drug reaches the baby but little of the levothyroxine does.
How each works
| Titration | Block and replace | |
|---|---|---|
| Tablets | Carbimazole (or methimazole) only | Carbimazole plus levothyroxine |
| Carbimazole dose | Starts higher, then lowered to the least that works, often 5 to 15 mg a day | Stays at a fixed blocking dose |
| Blood tests | Frequent while the dose is found | Fewer adjustments once settled |
| Typical length | 12 to 18 months | 12 to 18 months |
| Pregnancy | Can be used, under a specialist | Avoided |
| Children | Preferred by NICE | Not first choice |
The NHS describes block and replace as a combination of carbimazole and levothyroxine “to help find the right hormone balance”. NICE allows either regimen for adults with Graves’ disease, for 12 to 18 months, and recommends titration for children and young people.
What the evidence says
The Cochrane review pooled 12 trials comparing the two. Relapse after stopping was almost the same: 51% after block and replace and 54% after titration, a difference that could be chance. But block and replace caused more rashes (10% against 6%), and more people withdrew because of side effects (16% against 9%). The reviewers rated the trials as poor quality overall.
The likely reason for the extra side effects is dose. Many reactions to carbimazole and methimazole, including the rare blood disorder agranulocytosis, are more common at higher doses: in one Japanese hospital comparison agranulocytosis was about four times more frequent at 30 mg of methimazole a day than at 15 mg.
Who might choose which
Titration suits most people, especially anyone sensitive to side effects, anyone who may become pregnant, and children.
Block and replace can suit people whose levels swing up and down on titration, or who would rather have fewer dose changes and blood tests. Your specialist will weigh this against the higher dose.
Pregnancy
The Neo-Mercazole product information says block and replace should not be used in pregnancy, because very little thyroxine crosses the placenta while carbimazole does, so the baby’s thyroid could be blocked without replacement. Anyone on block and replace who is planning a pregnancy should talk to their doctor first. See pregnancy.
Getting your tablets
Whichever regimen you are on, order exactly what is prescribed. Carbimazole is listed by PockDose, Yasabai and VGR Australia. Doses and timing are on dosage.
Sources
Checked 15 October 2026.
- Antithyroid drug regimen for treating Graves' hyperthyroidism (Abraham et al., 2010) — Cochrane Database of Systematic Reviews
- Thyroid disease: assessment and management (NG145), recommendations 1.6.22 and 1.6.23 — National Institute for Health and Care Excellence
- Neo-Mercazole New Zealand data sheet, sections 4.2 and 4.6 — Medsafe (New Zealand)
- Common questions about carbimazole — NHS (UK)
- Methimazole-induced agranulocytosis is more frequent with 30 mg than 15 mg daily (Takata et al., 2009) — Thyroid