How often should I change my toothbrush?

It is about mechanics, not germs

The usual reason given for replacing a toothbrush is bacteria, and it is the weaker reason. Your mouth is already full of bacteria, and there is no good evidence that a brush stored in the open in a dry bathroom reinfects a healthy person with anything meaningful.

The real reason is that worn bristles stop working. Brushing depends on bristle tips that are rounded and stiff enough to flex into the gum margin and spring back. Once the filaments have bent permanently outward, they slide over the surface instead of reaching the sulcus, and plaque removal falls off measurably. Studies comparing worn and new brushes consistently find worn ones remove less plaque, and the drop starts well before the brush looks ruined.

Three months is a proxy, not a rule of physics. It is roughly how long a medium-soft filament survives twice-daily brushing at reasonable pressure. Brush harder and it is six weeks.

Replace it early when

• The bristles splay past the edge of the head, or the coloured indicator bristles have faded — that is what they are for
• You have had strep throat, oral thrush, a cold sore outbreak, or a mouth infection. Bin it and start fresh
• You have been ill and are immunocompromised or recovering from surgery
• It has been dropped in the sink, the toilet, or a bag
• It is a child's brush — children chew, and their brushes are often finished in 6–8 weeks
• You have been travelling with it sealed damp in a plastic case for days

A soft brush, and less pressure than you think

Almost everyone should be using a soft brush. Medium and hard bristles do not clean better; they abrade enamel at the gumline and drive gum recession, which is permanent. If your brush splays in a month, the brush is not the problem — the pressure is. Hold it in your fingertips rather than your fist.

Store it upright and uncovered so it dries between uses, not in a sealed travel cap. Rinse it after brushing, do not share it, and keep it a little away from the toilet if the bathroom layout allows. Sanitising it, boiling it, or microwaving it is unnecessary and shortens its life.

The habit that makes the interval stick

Because 3 months is exactly one quarter, the common trick is to tie replacement to the start of each season, or to a date you already remember. That works, and it is the reason toothbrush multipacks exist — the second brush in the drawer is what makes replacement a 10-second decision instead of a shopping trip.

Buy them four at a time and the only remaining question is when the current one went into service.