diff --git a/db/types.go b/db/types.go
index 534d6168..137753a4 100644
--- a/db/types.go
+++ b/db/types.go
@@ -11,6 +11,12 @@ type Beginner interface {
 	Begin() (*sql.Tx, error)
 }
 
+// Querier is an interface for types that can execute SQL queries.
+// *sql.DB, *sql.Tx, and *DB all implement this.
+type Querier interface {
+	Query(query string, args ...any) (*sql.Rows, error)
+}
+
 // Host pairs a *sql.DB with the host:port it was opened against.
 type Host struct {
 	Addr    string // "host:port"
diff --git a/user/manager.go b/user/manager.go
index 28243a24..76352384 100644
--- a/user/manager.go
+++ b/user/manager.go
@@ -288,33 +288,41 @@ func (a *Manager) ChangeTier(username, tier string) error {
 	t, err := a.Tier(tier)
 	if err != nil {
 		return err
-	} else if err := a.checkReservationsLimit(username, t.ReservationLimit); err != nil {
-		return err
 	}
-	if _, err := a.db.Exec(a.queries.updateUserTier, tier, username); err != nil {
-		return err
-	}
-	return nil
+	return db.ExecTx(a.db, func(tx *sql.Tx) error {
+		if err := a.checkReservationsLimitTx(tx, username, t.ReservationLimit); err != nil {
+			return err
+		}
+		if _, err := tx.Exec(a.queries.updateUserTier, tier, username); err != nil {
+			return err
+		}
+		return nil
+	})
 }
 
 // ResetTier removes the tier from the given user
 func (a *Manager) ResetTier(username string) error {
 	if !AllowedUsername(username) && username != Everyone && username != "" {
 		return ErrInvalidArgument
-	} else if err := a.checkReservationsLimit(username, 0); err != nil {
-		return err
 	}
-	_, err := a.db.Exec(a.queries.deleteUserTier, username)
-	return err
+	return db.ExecTx(a.db, func(tx *sql.Tx) error {
+		if err := a.checkReservationsLimitTx(tx, username, 0); err != nil {
+			return err
+		}
+		if _, err := tx.Exec(a.queries.deleteUserTier, username); err != nil {
+			return err
+		}
+		return nil
+	})
 }
 
-func (a *Manager) checkReservationsLimit(username string, reservationsLimit int64) error {
-	u, err := a.User(username)
+func (a *Manager) checkReservationsLimitTx(tx *sql.Tx, username string, reservationsLimit int64) error {
+	u, err := a.userTx(tx, username)
 	if err != nil {
 		return err
 	}
 	if u.Tier != nil && reservationsLimit < u.Tier.ReservationLimit {
-		reservations, err := a.Reservations(username)
+		reservations, err := a.reservationsTx(tx, username)
 		if err != nil {
 			return err
 		} else if int64(len(reservations)) > reservationsLimit {
@@ -388,7 +396,11 @@ func (a *Manager) writeUserStatsQueue() error {
 
 // User returns the user with the given username if it exists, or ErrUserNotFound otherwise
 func (a *Manager) User(username string) (*User, error) {
-	rows, err := a.db.Query(a.queries.selectUserByName, username)
+	return a.userTx(a.db, username)
+}
+
+func (a *Manager) userTx(tx db.Querier, username string) (*User, error) {
+	rows, err := tx.Query(a.queries.selectUserByName, username)
 	if err != nil {
 		return nil, err
 	}
@@ -753,7 +765,11 @@ func (a *Manager) RemoveReservations(username string, topics ...string) error {
 
 // Reservations returns all user-owned topics, and the associated everyone-access
 func (a *Manager) Reservations(username string) ([]Reservation, error) {
-	rows, err := a.db.ReadOnly().Query(a.queries.selectUserReservations, Everyone, username)
+	return a.reservationsTx(a.db.ReadOnly(), username)
+}
+
+func (a *Manager) reservationsTx(tx db.Querier, username string) ([]Reservation, error) {
+	rows, err := tx.Query(a.queries.selectUserReservations, Everyone, username)
 	if err != nil {
 		return nil, err
 	}